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The Physics And Chemistry Of The Interstellar Medium A G G M Tielens

The Physics And Chemistry Of The Interstellar Medium A G G M Tielens The Physics And Chemistry Of The Interstellar Medium A G G M Tielens The Physics And Chemistry Of The Interstellar Medium A G G M Tielens

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The Physics and Chemistry of the Interstellar Medium This work provides a comprehensive overview of our current theoretical and observational understanding of the interstellar medium of galaxies. With emphasis on the microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, the book includes the latest developments in this area of molecular astrophysics. The various heating, cooling, and chemical processes relevant for the rarefied gas and submicron-sized dust grains that constitute the interstellar medium are discussed in detail. This provides a firm foundation for an in-depth understanding of the ionized, neutral atomic, and molecular phases of the interstellar medium. The physical and chemical properties of large polycyclic aromatic hydrocarbon molecules and their role in the interstellar medium are highlighted, and the physics and chemistry of warm and dense photodissociation regions are discussed. This is an invaluable reference source for advanced undergraduate and graduate students and research scientists. Alexander Tielens is a professor of astrophysics at the Kapteyn Astronomical Institute in the Netherlands, and a senior scientist with the Dutch space agency, SRON. Prior to this, he has worked as an assistant researcher in the Astronomy Department of the University of California, Berkeley, and as a senior scientist at NASA Ames Research Center, California. He has published extensively on various aspects of the physics and chemistry of the interstellar medium.
THE PHYSICS AND CHEMISTRY OF THE INTERSTELLAR MEDIUM A.G.G.M. TIELENS Kapteyn Astronomical Institute
   Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge  , UK First published in print format - ---- - ---- © A. G. G. M. Tielens 2005 2005 Information on this title: www.cambridge.org/9780521826341 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. - --- - --- Cambridge University Press has no responsibility for the persistence or accuracy of s for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Published in the United States of America by Cambridge University Press, New York www.cambridge.org hardback eBook (NetLibrary) eBook (NetLibrary) hardback
Contents Preface page ix List of constants xi List of conversion factors xii 1 The galactic ecosystem 1 1.1 Interstellar objects 2 1.2 Components of the interstellar medium 6 1.3 Energy sources 12 1.4 The Milky Way 18 1.5 The mass budget of the ISM 20 1.6 The lifecycle of the Galaxy 22 1.7 Physics and chemistry of the ISM 22 1.8 Further reading 23 2 Gas cooling 25 2.1 Spectroscopy 25 2.2 Cooling rate 45 2.3 Two-level system 46 2.4 Gas cooling in ionized regions 53 2.5 Gas cooling in neutral atomic regions 54 2.6 Cooling law 58 2.7 Further reading 60 3 Gas heating 63 3.1 Overview 63 3.2 Photo-ionization of atoms 64 3.3 Photo-electric heating 66 3.4 Photon heating by H2 71 3.5 Dust-gas heating 74 3.6 Cosmic-ray heating 75 v
vi Contents 3.7 X-ray heating 76 3.8 Turbulent heating 77 3.9 Heating due to ambipolar diffusion 79 3.10 Gravitational heating 80 3.11 The heating of the interstellar medium 80 3.12 Further reading 83 4 Chemical processes 85 4.1 Gas-phase chemical reactions 85 4.2 Grain-surface chemistry 101 4.3 Further reading 114 5 Interstellar dust 117 5.1 Introduction 117 5.2 Physical processes 118 5.3 Observations 145 5.4 The sizes of interstellar grains 157 5.5 The composition of interstellar dust 161 5.6 Further reading 169 6 Interstellar polycyclic aromatic hydrocarbon molecules 173 6.1 Introduction 173 6.2 IR emission by PAH molecules 175 6.3 PAH charge 190 6.4 Photochemistry of PAHs 198 6.5 Other large molecules 209 6.6 Infrared observations 212 6.7 The IR characteristics of PAHs 213 6.8 Further reading 224 7 HII regions 228 7.1 Overview 228 7.2 Ionization balance 228 7.3 Energy balance 246 7.4 Emission characteristics 251 7.5 Comparison with observations 256 7.6 Further reading 263 8 The phases of the ISM 265 8.1 Introduction 265 8.2 Physical processes in atomic gas 266 8.3 The CNM and WNM phases of the ISM 271 8.4 The warm ionized medium 276
Contents vii 8.5 The hot intercloud medium 277 8.6 Summary: the violent ISM 285 8.7 Chemistry of diffuse clouds 287 8.8 The cosmic-ray ionization rate 299 8.9 Observations 304 8.10 Further reading 314 9 Photodissociation regions 317 9.1 Introduction 317 9.2 Ionization balance 319 9.3 Energy balance 320 9.4 Dust temperature 324 9.5 Chemistry 326 9.6 PDR structure 329 9.7 Comparison with observations 331 9.8 PDR diagnostic model diagrams 336 9.9 The Orion Bar 339 9.10 Physical conditions in PDRs 343 9.11 Hydrogen IR fluorescence spectrum 344 9.12 Further reading 345 10 Molecular clouds 348 10.1 Introduction 348 10.2 The degree of ionization 349 10.3 Energy balance 351 10.4 Gas-phase chemistry 353 10.5 Grain-surface chemistry 364 10.6 Gas–grain interaction 369 10.7 Observations 377 10.8 Further reading 392 11 Interstellar shocks 396 11.1 Introduction 396 11.2 J-shocks 397 11.3 C-shocks 408 11.4 Further reading 416 12 Dynamics of the interstellar medium 417 12.1 Introduction 417 12.2 The expansion of HII regions 418 12.3 Supernova explosions 440 12.4 Supernovae and the interstellar medium 447 12.5 Interstellar winds 453
viii Contents 12.6 The kinetic energy budget of the ISM 458 12.7 Further reading 459 13 The lifecycle of interstellar dust 461 13.1 Introduction 461 13.2 Shock destruction 463 13.3 Dust lifetimes 468 13.4 The grain size distribution 470 13.5 Dust abundances and depletions 470 13.6 Mass balance of interstellar dust 474 13.7 Further reading 474 14 List of symbols 476 Index of compounds 484 Index of polycyclic aromatic hydrocarbons 485 Index of molecules 487 Index of objects 490 Index 491
Preface When, upon my return to Holland, I started to teach an advanced course on the interstellar medium in 1998, I quickly realized that there was no suitable textbook available. There is, of course, the incomparable monograph by Spitzer, Physics of the Interstellar Medium (1978, New York: Wiley and Sons). But that book is quite challenging and not very suitable for a student course. Moreover, by now, it is very dated. Over the intervening years, our insights into the basic physics of the interstellar medium have much improved thanks, for example, to the opening up of the infrared and submillimeter windows. In particular, molecules, which we now know to be deeply interwoven into the fabric of the Universe, play only a little role in Spitzer’s book. When Eddington made his famous remark, “Atoms are physics but molecules are chemistry,” he merely expressed, on the one hand, the dream of a physicist of a simple universe, which can be caught in a single equation, and, on the other hand, the dread of a reality where solutions are never clean and simple. The latter is of course obvious to a chemist and it is now abundantly clear that Eddington’s fear has turned into reality, even for astronomy. Present-day graduate students will require an intimate knowledge of molecular astrophysics in order to be active in the field of the interstellar medium of our own or other galaxies whether it is in the here and now or all the way back in the early Universe. This will become even more the case with the launch of the submillimeter space mission, Herschel, in 2007, when the Atacama Large Millimeter Array is finished in 2011, and with the launch of the James Webb Space Telescope in the next decade. Together these missions will push the frontier of the molecular Universe all the way back to the initial pollution of the Universe with the first metals by the first generation of luminous objects, which forever spoiled the physicist’s Garden of Eden. This book covers both the physics and the chemistry of the interstellar medium. Chapters on heating, cooling, and chemical processes provide the students with the necessary toolbox for the astrophysics and astrochemistry of the interstel- lar medium. This background is rounded off with chapters on the physics and ix
x Preface chemistry of interstellar dust and large molecules. Once the students have mastered these subjects, they are well prepared for an in-depth discussion of classical topics of the interstellar medium: HII regions, the phases of the interstellar medium, shocks, and the dynamical interaction of HII regions, supernova remnants, and stellar winds with the ISM. The chemistry of the interstellar medium is covered in chapters on diffuse clouds, photodissociation regions, and molecular clouds. All together, this forms a comprehensive course, covering most current aspects of the interstellar medium, which will prepare students well for the future. Over the years, this book grew from the course that I taught in Groningen. Indeed, in many ways, writing this book carried me through those dark Dutch days. Fortunately, I have many good friends who understand that, when the Sun sets in October not to appear again until May, it is a good time to leave Holland and visit other institutes. I owe a deep debt of gratitude to the Miller Institute and the Astronomy Department of the University of California in Berkeley and my hosts Imke de Pater and Chris McKee, to the Space Sciences Division of NASA Ames Research Center and my hosts David Hollenbach and Lou Allamandola, to the Institute for Geophysics and Planetary Physics of the Lawrence Livermore National Laboratory and my hosts Wil van Breugel and John Bradley, to the Centre d’Etudes Spatiale des Rayonnements in Toulouse and my host Emmanuel Caux, and to the Laboratoire d’Astrophysique de l’Observatoire de Grenoble and my host Cecilia Ceccarelli for their hospitality and for providing an environment conducive to great science. Most of the chapters of this book were conceived and written during these extended visits. Of course, much of this book reflects a lifetime spent in discovering the molecular Universe. I want to thank Harm Habing without whose human touch I would have left astronomy before even finishing the first stage of my journey. Also, I owe much to Lou Allamandola and David Hollenbach, with whom I have spent so many wonderful hours on the trail of discovery: not only for sharing their deep insights and understanding of physical and chemical processes of relevance to studies of the interstellar medium but, particularly, for their friendship. I am also deeply indebted to the many graduate students, who carried me through the many stages of this course, solved the many L ATEX problems, and always succeeded in making the right figures, as well as for their careful proofreading of the manuscript. Most of all, their enthusiasm always managed to perk me up. In this regard, I specifically want to thank Adwin Boogert, Rense Boomsma, Jan Cami, Stephanie Cazaux, Sasha Hony, Jacquie Keane, Leticia Martín-Hernández, Chris Ormel, Els Peeters, and Henrik Spoon for their help. Finally, Marion understands like no other what it is to live away from what feels like home. Her encouragement to follow my dream and her support during these difficult years have made this possible. To my girls–Anneke, Saskia, and Elske–the only thing I can say is that, now, it is really done.
Constants Physical constants Symbol Description SI cgs Value Unit Value Unit c Speed of light 29979 8 m s−1 29979 10 cm−1 s−1 h Planck’s constant 66261−34 J s 66261−27 erg s k Boltzmann’s constant 13807−23 J/K 13807−16 erg/K SB Stefan–Boltzmann constant 56704 −8 W m−2 K−4 56704 −5 erg s−1 cm−2 K−4 G Gravitational constant 6674 −11 N m−2 kg−2 6674 −8 dyn cm−2 g−2 NA Avogadro’s constant 60221 23 mol−1 60221 23 mol−1 me Electron rest mass 91094−31 kg 91094−28 g mp Proton rest mass 16726−27 kg 16726−24 g mu Atomic mass unit 16605−27 kg 16605−24 g e Electron charge 1602 −19 C 4803 −10 esu Fine-structure constant 72974 −3 72974 −3 Values a×10b are given as a (b). Astronomical constants Symbol Description SI cgs Value Unit Value Unit AU Astronomical unit 1496 11 m 1496 13 cm ly Light year 9463 15 m 9463 17 cm pc Parsec 3086 16 m 3086 18 cm pc2 Square parsec 95234 32 m2 95234 36 cm2 kpc2 Square kiloparsec 95234 38 m2 95234 42 cm2 L Solar luminosity 385 26 J s−1 385 33 erg s−1 M Solar mass 1989 30 kg 1989 33 g R Solar radius 696 8 m 696 10 cm T Solar effective temperature 578 3 K 578 3 K Jy Jansky 100 −26 W m−2 H z−1 100 −23 erg s−1 cm−2 Hz−1 Values a×10b are given as a (b). xi
Conversion factors Angles and lengths Unit/symbol Description SI cgs Value Unit Value Unit deg degree 17453 −2 rad 17453 −2 rad arcmin arcminute 290888−4 rad 290888−4 rad arcsec arcsecond 48481 −6 rad 48481 −6 rad sq deg degree2 3046 −4 sr 3046 −4 sr Å angstrom 10 −10 m 10 −8 cm m micrometer 10 −6 m 10 −4 cm Values a×10b are given as a (b). SI and cgs units Description SI cgs Value Unit Value Unit Time 1 s 1 s 1 year 3.16 (7) s Length 1 m 1 (2) cm Velocity 1 m s−1 1 (2) cm s−1 Force 1 N 1 (5) dyne Pressure 1 Pa 1 (−1) dyne cm−2 Energy 1 J 1 (7) erg Charge 1 C 2.9979 (9) esu Magnetic flux density 1 T 1 (4) gauss Values a×10b are given as a (b). xii
List of conversion factors xiii Energy conversion factors erg eV K cm−1 Hz erg 100 6242 11 7243 15 5034 15 1509 26 eV 1602 −12 100 11604 4 80644 2418 14 K 13806−16 8617 −5 100 0695 2084 10 cm−1 19865−16 1240 −4 14389 100 2997010 Hz 6626 −27 4136−15 4798 −11 3336−11 100 Values a × 10b are given as a (b). To convert from unit in column 1 to units above the rows, multiply by value; e.g., 1eV = 1602×10−12 erg. A useful compendium of constants can be found in C. W. Allen, Astrophysical Quantities, (London: The Athlone Press). The website http://physics.nist.gov/cuu/, maintained by the National Institute of Standards and Technology, provides a wealth of information on constants.
1 The galactic ecosystem The Milky Way is largely empty. Stars are separated by some 2 pc in the solar neighborhood = 6 × 10−2 pc−3. If we take our Solar System as a measure, with a heliosphere radius of 235 AU, stars and their associated planetary systems fill about 3×10−10 of the available space. This book deals with what is in between these stars: the interstellar medium (ISM). The ISM is filled with a tenuous hydrogen and helium gas and a sprinkling of heavier atoms. These elements can be neutral, ionized, or in molecular form and in the gas phase or in the solid state. This gas and dust is visibly present in a variety of distinct objects: HII regions, reflection nebulae, dark clouds, and supernova remnants. In a more general sense, the gas is organized in phases – cold molecular clouds, cool HI clouds, warm intercloud gas, and hot coronal gas – of which those objects are highly visible manifestations. This gas and dust is heated by stellar photons, originating from many stars (the so-called average interstellar radiation field), cosmic rays (energetic [∼GeV] protons), and X-rays (emitted by local, galactic, and extragalactic hot gas). This gas and dust cools through a variety of line and continuum processes and the spectrum will depend on the local physical conditions. Surveys in different wavelength regions therefore probe different components of the ISM. This first chapter presents an inventory of the ISM with an emphasis on prominent objects in the ISM and the global structure of the ISM. The interstellar medium plays a central role in the evolution of the Galaxy. It is the repository of the ashes of previous generations of stars enriched by the nucleosynthetic products of the fiery cauldrons in the stellar interiors. These are injected either with a bang, in a supernova explosion, or with a whimper, in the much slower moving winds of low-mass stars on the asymptotic giant branch. In this way, the abundances of heavy elements in the ISM slowly increase. This is part of the cycle of life for the stars of the Galaxy, because the ISM itself is the birthplace of future generations of stars. It is this constant recycling and its 1
2 The galactic ecosystem Figure 1.1 A panoramic image of a (southern) portion of the Milky Way’s disk. The image has been inverted and dark corresponds to emission from ionized gas and reflection nebulae. The light band stretching irregularly across the whole image is due to absorption by dust clouds. Image courtesy of J. P. Gleason. associated enrichment that drives the evolution of the Galaxy, both physically and in its emission characteristics. 1.1 Interstellar objects 1.1.1 HII regions Ionized gas nebulae feature prominently in the Milky Way as bright visible nebulous objects. The Great Nebula in Orion (M42; Fig. 1.2) and the Lagoon Nebula (M8) are well-known examples. HII regions span a range in brightness, however, and fainter examples are the California Nebula and IC 434 (Fig. 1.3). The gas in these regions is ionized and has a temperature of about 104 K. Densities range from 103 –104 cm−3 for compact (∼05 pc) HII regions such as the Orion Nebula to ∼10 cm−3 for more diffuse and extended nebulae such as the North America Nebula (∼10 pc). The optical spectra of these regions are dominated by H and He recombination lines and collisionally excited, optical (forbidden) line emission from trace ions such as [OII], [OIII], and [NII]. HII regions are also strong sources of thermal radio emission (free–free) from the ionized gas and of infrared emission due to warm dust. HII regions are formed by young massive stars with spectral type earlier than about B1 (Teff 25000 K), which emit copious amounts of photons beyond the Lyman limit (h 136 eV) and ionize and heat their surrounding, nascent molecular clouds. They are, therefore, signposts of sites of massive star formation in the Galaxy. 1.1.2 Reflection nebulae Reflection nebulae are bluish nebulae that reflect the light of a nearby bright star. NGC 2023 in the Orion constellation (see Fig. 1.3) and the striated nebulosity associated with the Pleiades are familiar cases. In this case, the observed light
1.1 Interstellar objects 3 Figure 1.2 A black and white representation of the Orion Nebula as observed by the Hubble Space Telescope in [OIII], H, and [NII]. Light and dark have been inverted. The gas is ionized by the Trapezium cluster, in particular 1 C Ori, in the center of the image. The bright bar in the south-west is an ionization front eating its way into the surrounding neutral material in the photodissoci- ation region known as the Orion Bar. The dark bay (light in this image), a cloud of foreground obscuring material, is also evident to the east. This image gives a clear view of the complex topography created by the interaction of a newly formed massive star with its surrounding natal cloud. Image courtesy of R. O’Dell. is not due to hot gas but rather reflected starlight. There is no radio emission but there is infrared emission from warm dust, although this is less luminous than for HII regions. For the compact reflection nebulae, densities are typically a little smaller (103 cm−3 ) than for compact HII regions. Reflection nebulae are illuminated by stars with spectral types later than about B1. Regions around hotter stars also show (faint) reflected light emission but the spectrum is then dominated by the emission from the ionized gas. For the earlier stellar types, the surrounding nebulosity may be the material from which the star was formed (e.g., NGC 2023; NGC 7023). Often, however, the nebulosity is due to a chance encounter between the star and a cloud (e.g., the Pleiades). Reflection nebulae can also be associated with the ejecta of a late-type star (e.g., IC 2220; the Red Rectangle).
4 The galactic ecosystem Figure 1.3 Part of the Orion molecular cloud containing the Horse Head Nebula. The diffuse glow behind the horse head is IC 434 ionized by the bright star, Ori. The horse head is a protrusion of the molecular cloud obvious in the lower part of this image. The nebula to the south-east of the horse head is the reflection nebula, NGC 2023. Image courtesy of the Canadian-France-Hawaii telescope, J.-C. Cuillandre, Coelem. 1.1.3 Dark nebulae A striking aspect of the all-sky optical view of the Milky Way is the presence of many dark regions in which few stars are seen (cf. Fig. 1.1). The direction towards the center of the Galaxy is rampant with such dark clouds, which actually seem to divide the galactic plane in two. The Coalsack near the Southern Cross is a particularly nice example of a roundish dark cloud. Dark clouds are readily apparent when backlighted. The Horse Head Nebula (see Fig. 1.3) silhouetted against the reddish glow of the HII region, IC 434, and the dark bay in the Orion HII region (see Fig. 1.2) are two famous examples. Individual dark clouds come in a range of sizes from tens-of-parsecs large to the tiny (∼10−2 pc) Bok globules associated with HII regions such as the Orion Nebula. Likewise, some dark clouds are completely black (Av 10 magnitudes) while others are hardly discernible. While dark clouds are outlined by the absence of stars, they do show faint optical
1.1 Interstellar objects 5 reflected light. Also, they become bright at mid- and far-infrared wavelengths. Some really dense clouds are opaque even at mid-IR wavelengths and appear as infrared dark clouds (IRDC) in absorption against background galactic mid-IR emission. 1.1.4 Photodissociation regions While HII regions and reflection nebulae dominate the Galaxy at visible wave- lengths, in the infrared, photodissociation regions dominate the sky. Originally, the name photodissociation regions (PDRs; sometimes also called photodominated regions with fortunately the same abbreviation) was given to the atomic–molecular zones that separate ionized and molecular gas near bright luminous O and B stars (e.g., surrounding HII regions and reflection nebulae) and the Orion Bar (Fig. 1.2) and NGC 2023 (Fig. 1.3) are prime examples of classical PDRs. In these regions, penetrating far-ultraviolet (FUV) photons (with energies between 6 and 13.6 eV) dissociate and ionize molecular species. Most of the FUV photons are absorbed by the dust, but a small fraction heat the gas through the photo- electric effect to a few hundred degrees. Photodissociation regions are thus bright in IR dust continuum and atomic fine-structure cooling lines as well as molecular lines. In essence, of course, everywhere where FUV photons strike a cloud, a PDR will ensue. Indeed, the term PDRs has now expanded to include all regions of the ISM where FUV photons dominate the physical and chem- ical processes. As such, PDRs include the neutral atomic gas of the ISM as well as much of the gas in molecular clouds (except, e.g., for dense starless cores). 1.1.5 Supernova remnants Supernova remnants (SNRs) are formed when the material ejected in the explosion that terminates the life of some stars shocks surrounding ISM material and an SNR’s spectrum is that of a high velocity shock. About 100 supernova remnants are visible in our Galaxy; they are generally characterized by long, delicate fila- ments radiating in-line radiation (Fig. 1.4). Supernova remnants are prominent sources of radio emission due to relativistic electrons spiraling around a magnetic field (synchroton emission) and some 200 have been identified at radio wave- lengths. Supernova remnants also stick out at X-ray wavelengths because of emission by hot (106 K) gas. Not all SNRs are wispy. The Crab Nebula is an example of a compact SNR.
6 The galactic ecosystem Figure 1.4 A small portion of the Cygnus Loop, the remnant of a supernova that exploded about 10 000 years ago. The image has been inverted to bring out the delicate structure of the nebulosity. The emission is due to a shock wave and is about 3 pc in size. Image courtesy of L. K. Tan, StarryScapes. 1.2 Components of the interstellar medium The gas in the ISM is organized in a variety of phases. The physical properties of these phases are summarized in Table 1.1. 1.2.1 Neutral atomic gas The 21 cm line of atomic hydrogen traces the neutral gas of the ISM. This neutral gas can also be observed in optical and UV absorption lines of various elements towards bright background stars. The neutral medium is organized in cold ( 100 K) diffuse HI clouds (cold neutral medium, CNM) and warm (≈8000 K) intercloud gas (warm neutral medium, WNM). A standard HI cloud (often called a Spitzer-type cloud) has a typical density of 50 cm−3 and a size of 10 pc. The density of the WNM is much less (05 cm−3). Between 4 and 8 kpc from the galactic center, 80% of the HI mass in the plane of the Galaxy is in diffuse clouds in a layer with a (Gaussian) scale height of about 100 pc. At higher latitudes, however, much of the HI mass is in the intercloud medium with a larger scale height of 220 pc but with an exponential tail extending well into the lower halo. These two neutral phases have, on average, similar surface densities. Because the Sun is located in the local bubble, the local, total WNM column density towards the North Galactic Pole is about 2.5 times that of the CNM. In the outer Galaxy, the HI scale height rapidly increases.
1.2 Components of the interstellar medium 7 Table 1.1 Characteristics of the phases of the interstellar medium Phase na 0 (cm−3 ) Tb (K) c v (%) Md (109 M) n0 e (cm−3 ) Hf (pc) g Mpc−2 Hot intercloud 0.003 106 ∼50.0 — 00015 3000 03 Warm neutral medium 0.5 8000 30.0 28 01h 220h 15 006h 400h 14 Warm ionized medium 0.1 8000 25.0 10 0025i 900i 11 Cold neutral mediumj 50.0 80 1.0 22 04 94 23 Molecular clouds 200.0 10 0.05 13 012 75 10 HII regions 1–105 104 — 005 0015k 70k 005 a Typical gas density for each phase. b Typical gas temperature for each phase. c Volume filling factor (very uncertain and controversial!) of each phase. d Total mass. e Average mid-plane density. f Gaussian scale height, ∼ exp −z/H2 /2, unless otherwise indicated. g Surface density in the solar neighborhood. h Best represented by a Gaussian and an exponential. i WIM represented by an exponential. j Diffuse clouds. k HII regions represented by an exponential. 1.2.2 Ionized gas Diffuse ionized gas in the ISM can be traced through dispersion of pulsar signals, through optical and UV ionic absorption lines against background sources, and through emission in the H recombination line (see Fig. 1.5). The first two can only be done in a limited number of selected sight-lines. The faintness and large extent of the galactic H hamper the last probe. While most of the H luminosity of the Milky Way is emitted by distinct HII regions, almost all of the mass of ionized gas (109 M) resides in a diffuse component. This warm ionized medium (WIM) has a low density (01 cm−3 ), a temperature of ≈8000 K, a volume filling factor of 025, and a scale height of 1 kpc. The weakness of the [OI] 6300 line (in a few selected directions) implies that the gas is nearly fully ionized. The source of ionization is not entirely clear. Energetically, ionizing photons from O stars are the most likely candidates but these photons have to “escape” from the associated HII regions and travel over large distances (hundreds of parsecs)
8 The galactic ecosystem Wisconsin H-Alpha Mapper Northern Sky Survey Total Integrated Intensity Map (–80 VLSR +80 km s–1 ) l=120° ∆l=30° Log Intensity [Rayleighs] http://www.astro.wisc.edu/wham/ 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 –0.25 –0.50 ∆b=15° Figure 1.5 The integrated galactic ( versus b) H emission obtained by the WHAM survey. This emission from ionized gas, about a million times fainter than the Orion Nebula, traces the warm ionized medium. Note the large filament (40 ) sticking up out of the galactic plane. Image courtesy of R. Reynolds. without being absorbed by omnipresent neutral hydrogen. Finally, the WIM shows a complex spatial structure including thin filaments sticking ∼1 kpc out of the plane of the Galaxy, further compounding the ionization problem. 1.2.3 Molecular gas The CO J = 1−0 transition at 2.6 mm is commonly used as a tracer of molecular gas in the Galaxy (Fig. 1.6). Surveys in this line have shown that much of the molecular gas in the Milky Way is localized in discrete giant molecular clouds with typical sizes of 40 pc, masses of 4×105 M, densities of 200 cm−3 , and temperatures of 10 K. However, it should be understood that molecular clouds show a large range in each of these properties. Molecular clouds are characterized by high turbulent pressures as indicated by the large linewidths of emission lines. Molecular clouds are self-gravitating rather than in pressure equilibrium with other phases in the ISM. While they are stable over time scales of 3×107 years, presumably because of a balance of magnetic and turbulent pressure and gravity, molecular clouds are the sites of active star formation. Observations of molecular
1.2 Components of the interstellar medium 9 Figure 1.6 The emission of CO in the outer Galaxy obtained by the FCRAO survey. Much of the emission is local, stemming from molecular clouds between 0.5 and 1 kpc. In addition, giant molecular clouds associated with the NGC 7538-Sharpless 156-Sharpless 152 region ( = 110 ) and the W3-W4-W5 region ( = 135 ) are also present. The image has been inverted and dark corresponds to emission. Image courtesy of Chris Brunt (FCRAO). clouds in the rotational transitions of a variety of species allow a detailed study of their physical and chemical properties. These studies show that molecular clouds have spatial structure on all scales. In particular, molecular clouds contain cores with sizes of 1 pc, densities in excess of 104 cm−3 , and masses in the range 10–103 M in which star formation is localized. While CO is commonly used to trace interstellar molecular gas, H2 is thought to be the dominant molecular species, with a H2/CO ratio of 104 –105 . In addition, some 200 different molecular species have been detected – mainly through their rotational transitions in the submillimeter wavelength regions – in the shielded environments of molecular clouds. In general, these species are relatively simple, often unsaturated, radi- cals, or ions. Acetylenic carbon chains and their derivatives figure prominently on the list of detected molecules. However, this may merely reflect observa- tional bias, since such species possess large dipole moments and relatively small partition functions, both of which make them readily detectable at microwave wavelengths. 1.2.4 Coronal gas Hot (∼105 –106 K) gas can be traced through UV absorption lines of highly ionized species (e.g., CIV, SVI, NV, OVI) seen against bright background sources. Such hot plasmas also emit continuum (bremsstrahlung, radiative recombination, two photon) and line (collisionally excited and recombination) radiation in the extreme ultraviolet and X-ray wavelength regions. These observations have revealed the existence of a pervasive hot (3–10 × 105 K) and tenuous (10−3 cm−3) phase (the hot intercloud medium, HIM) of the ISM. The observations indicate a range of temperatures where the higher ionization stages probe hotter gas. The hot gas fills most of the volume of the halo (scale height 3 kpc) but the volume filling factor
10 The galactic ecosystem in the disk is more controversial. This gas is heated and ionized through shocks driven by stellar winds from early type stars and by supernova explosions. Much of the hot, high-latitude gas may have been vented by superbubbles created by the concerted efforts of whole OB associations into the halo in the form of a galactic fountain. This hot gas cools down, “condenses” into clouds, and rains down again on the disk. In the disk, the distribution of the hot gas is quite irregular. The Sun itself is located in a hot bubble with a size of approximately 100 pc. 1.2.5 Interstellar dust The presence of dust in the interstellar medium manifests itself in various ways. Through their absorption and scattering, small dust grains give rise to a general reddening and extinction of the light from distant stars (Fig. 1.1). Moreover, polar- ization of starlight is caused by elongated large dust grains aligned in the galactic magnetic field (dichroic absorption). Furthermore, near bright stars, scattering of starlight by dust produces a reflection nebula. Finally, the interstellar medium is bright in the infrared because of continuum emission by cold dust grains. Analysis of the wavelength dependence of interstellar reddening implies a size distribution, na ∼ a−35 , which ranges from 3000 Å all the way to the molecular domain (∼5 Å). The number density of grains with sizes ∼1000 Å is 10−13 per H atom. Most of the mass of interstellar dust is thus in the larger grains but the surface area is in the smallest grains. Abundance studies in the ISM show that many of the refractory elements (e.g., C, Si, Mg, Fe, Al, Ti, Ca) are locked up in dust; i.e., dust contains about 1% by mass of the gas. Large (100 Å) interstellar dust grains are in radiative equilibrium with the interstellar radiation field at temperatures of 15 K and the absorbed stellar photons are reradiated as infrared and submillimeter continuum emission. Near bright stars, the dust temperature is higher; typically some 75 K for a compact HII region. Emission by interstellar dust dominates these wavelength regions. Rotating interstellar dust grains also give rise to emission at radio wavelengths. Very small ( 100 Å) dust grains undergo fluctuations in their temperature upon the absorption of a single UV photon and emit at mid-IR (25–60 m) wavelengths. 1.2.6 Large interstellar molecules Besides dust grains, the interstellar medium also contains a population of large molecules. These molecules are particularly “visible” at mid-IR wavelengths. The IR spectrum of most objects – HII regions, reflection nebulae, surfaces of dark clouds, diffuse interstellar clouds and cirrus clouds, galactic nuclei, the interstellar
1.2 Components of the interstellar medium 11 Figure 1.7 An infrared view of the Eagle Nebula (image inverted) obtained with ISOCAM on the Infrared Space Observatory. The dark emission that dominates the image is due to IR fluorescence of large polycyclic aromatic hydrocarbon molecules. Near the center of the image some of the faint emission is due to large, cool dust grains present in the ionized gas. The well-known pillars are visible just under the center of this image as the three-fingered hand. Image courtesy of the ISOGAL team, especially Andrea Moneti and Frederic Schuller. medium of galaxies as a whole, and star burst galaxies – are dominated by broad infrared emission features (Fig. 1.7). These IR emission features are characteristics for polycyclic aromatic hydrocarbon (PAH) materials. These bands represent the vibrational relaxation process of FUV-pumped PAH species, containing some 50 C atoms. These species are very abundant, ∼10−7 relative to H, locking up about 10% of the elemental carbon. These PAHs may just be one – very visible – representative of the molecular Universe. In fact, visible spectra of stars generally show prominent absorption features that are too broad to be atomic in origin. These so-called diffuse interstel- lar bands (DIBs) are generally attributed to electronic absorption by moderately large molecules (10–50 C atoms). Unsaturated carbon chains, containing 10–20 C atoms, are leading candidates for these DIBs. There are now in excess of 200 DIBs known, of which some 50 are moderately strong. Because, typically, the visible spectrum of a molecular species is dominated by at most one strong transition, the DIBs implicate the presence of a large number of different molecular species. These large molecules seem to represent the extension of the interstellar grain size distribution into the molecular domain. Interstellar grains are known to contain
12 The galactic ecosystem several populations of nano particles: nano diamonds have been isolated from mete- orites with an isotopic composition that indicates a presolar origin; i.e., these grains predate the formation of the Solar System and they never fully equilibrated with the gas in the early solar nebula. Likewise, silicon nanoparticles may be the carrier of a widespread luminescence phenomena, the so-called extended red emission (ERE). 1.3 Energy sources 1.3.1 Radiation fields The ISM is permeated by various photon fields, which influence the physical and chemical state of the gas and dust (Fig. 1.8). The stellar radiation field contains contributions from early-type stars, which dominate the far-ultraviolet (FUV) cool stars PAHs dust CMB OB stars 10–14 10–16 10–18 10–20 10–22 10–24 10–26 10–7 10–5 10–3 10–1 101 Intensity λ(cm) hot gas Figure 1.8 The mean intensity in units of erg cm−2 s−1 Hz−1 sr−1 of the inter- stellar radiation field in the solar neighborhood. Contributions by hot gas, OB stars, older stars, large molecules (PAHs), dust, and the cosmic microwave back- ground are indicated. Figure adapted from J. Black, 1996, First Symposium on the IR Cirrus and Diffuse Interstellar Clouds, ed. R. M. Cutri and W. B. Latter (San Francisco: ASP), p. 355. The calculated X-ray/EUV emission spectrum and the FUV spectrum were kindly provided by J. Slavin. The dust emission is a fit to the COBE results for the galactic emission. The PAH spectrum was taken from ISO measurements of the mid-IR emission spectrum of the interstellar medium scaled to the measurements of the IR cirrus by IRAS (F. Boulanger, 2000, in ISO Beyond Point Sources: Studies of Extended Infrared Emission, ed. R. J. Laureijs, K. Leech, and M. F. Kessler, E. S. A.-S. P., 455, p. 3). The black squares at 12, 25, 60 and 100 m are the IRAS measurements of the IR cirrus, the DIRBE/COM measurement at 240 m, and those at 3.3, 3.5, and 4.95 m are the balloon measurement by Proneas experiment (M. Giard, J. M. Lamarre, F. Pajot, and G. Serra, 1994, A. A., 286, p. 203). Note that the latter have been superimposed on the stellar spectrum.
1.3 Energy sources 13 wavelengths, A-type stars, which control the visible region, and late-type stars, which are important at far-red to near-infrared wavelengths. The strength of the FUV average interstellar radiation is often expressed in terms of the Habing field, 12×10−4 erg cm−2 s−1 sr−1, named after Harm Habing, a pioneer in this field. Current estimates put the average interstellar radiation field at G0 = 17 Habing fields. Often, the radiation field in PDRs produced by a nearby star shining on a nearby cloud is expressed in terms of the equivalent one-dimensional average interstellar radiation field flux (e.g., 16×10−3 erg cm−2 s−1 ). These stellar photons are absorbed by dust grains and reradiated at longer wavelengths – in discrete emission bands in the mid-IR and in continuum emis- sion in the far-IR and submillimeter regions (Fig. 1.8). The 2.7 K cosmological background takes over at millimeter wavelengths. At extreme ultraviolet wave- lengths (EUV), even a small amount of neutral hydrogen absorbs all radiation and the intensity of the average radiation field due to stars drops precipitously at the Lyman edge (912 Å; Fig. 1.9). Stars do not contribute much at the shortest wave- lengths (X-rays). Instead, emission by hot plasmas – the coronal gas in the halo and in SNRs – dominates the radiation field. This component shows numerous emission lines. There is also an extragalactic contribution at the hardest energies. These X-ray emission components are mediated by absorption by foreground 101 10– 4 102 I λ (photons cm –2 s –1 Å –1 ) 10– 2 100 102 Interstellar radiation field OB stars Hot gas Older stars 104 106 103 λ (Å) 104 105 Figure 1.9 The average photon field in the solar neighborhood in units of photons cm−2 s−1 Å−1 . Contributions by hot gas, OB stars, and older stars are indicated. The calculated X-ray/EUV/FUV spectrum was kindly provided by J. Slavin. The visual spectrum was taken from J. S. Mathis, P. G. Mezger, and N. Panagia, 1983, A. A., 128, p. 212.
14 The galactic ecosystem Table 1.2 Energy balance Source P Energy density Heating rate (10−12 dyne cm−2 ) (eV cm−3 ) (erg s−1 H-atom−1 ) Thermal 0.5 60 –5 (–26)a UV — 05 5 (–26) Cosmic ray 1.0 20 3 (–27) Magnetic fields 1.0 06 2 (–27) Turbulence 0.8 15 1 (–27) 2.7 K background — 025 — a Energy loss rate. gas and, because the ionization cross section decreases rapidly with increasing energy at EUV–X-ray wavelengths, this effect is more pronounced at the longer wavelengths. Photodissociation rates and ionization rates are proportional to the photon intensity (Fig. 1.9). A simple polynomial fit for the mean photon intensity of the interstellar radiation field is ISRF = 8530×10−5 −1 −1376×10−1 −2 +5495×101 −3 cm−2 s−1 Hz−1 sr−1 (1.1) with in Å. The Habing field corresponds to about 108 photons cm−2 s−1 between 6 and 13.6 eV and about a factor of 10 fewer between 11 and 13.6 eV. 1.3.2 Magnetic fields The magnetic field is an important energy and pressure source in the ISM (cf. Table 1.2), controlling to a large extent the dynamics of the gas. The interstellar magnetic field manifests itself through linear polarization of starlight by aligned dust grains (dichroic absorption; Fig. 1.10), in polarization of far-infrared contin- uum emission of aligned dust grains, linear polarization of synchroton emission, Faraday rotation of background, polarized, radio sources, and Zeeman splitting of the 21 cm HI line and lines of molecules with unpaired electrons such as OH. The magnetic field is about 5 G in the solar neighborhood, increasing to about 8 G in the molecular ring at 4 kpc. Models for the synchroton emission, which trace the distribution of the magnetic field, imply the existence of a thin- disk component, associated with the gaseous disk, and a thick-disk component – the halo component – with a scale height of 1.5 kpc near the solar circle. The magnetic field consists of a uniform component and a non-uniform component. The uniform component of the magnetic field is roughly circular with a strength
1.3 Energy sources 15 Figure 1.10 The direction of the magnetic field of the Galaxy as measured through optical polarization of starlight. Upper panel: stars within 400 pc, empha- sizing the magnetic field associated with local clouds. Lower panel: stars between 2 and 4 kpc, showing some regions with magnetic field parallel to the galactic plane while others are more random. Figure reproduced with permission from D. S. Mathewson and V. L. Ford, 1970, Mem. R. A. S., 74, p. 139. of about 1.5 G in the solar neighborhood and has two field reversals within the solar circle and one outside. The field may show a spiral structure where the reversals occur in the interarm regions. There is also a considerable non-uniform magnetic field, partly associated with expanding interstellar shells (superbubbles) and their shocks. The strength of the magnetic field increases inside dense clouds, B ∼ n with 05 and typically B 30G at n 104 cm−3 . The direction of the field is correlated over the entire extent of the cloud from the diffuser outer parts to the denser cores. 1.3.3 Cosmic rays High energy ( 100 MeV nucleon−1) particles contribute considerably to the energy density of the ISM (2 eV cm−3; Table 1.2). Cosmic rays consist mainly of relativistic protons with energies between 1 and 10 GeV, 10% helium, and
16 The galactic ecosystem heavier elements and electrons at about the 1% level. The relative abundance of the elements in the cosmic rays is non-solar, attesting to the importance of spallation–production of light elements and an origin either in material of stel- lar (perhaps SN) composition or in sputtered interstellar grains. The interaction of energetic (1–10 GeV) cosmic-ray protons with interstellar gas gives rise to gamma rays with E 50 MeV through 0 meson decay emission. Likewise, the interaction of energetic (1 GeV) electrons with interstellar gas gives rise to gamma rays through bremsstrahlung and inverse Compton scattering. Gamma ray observations, in combination with interstellar gas surveys, can therefore be used to measure the distribution of cosmic rays in the Galaxy. Cosmic rays are tied to the galactic magnetic field and confined to a disk of radius 12 kpc with a thickness of ∼2 kpc. Cosmic rays seem to draw their energy from supernovae with an efficiency of about 10% of the kinetic energy of the ejecta. The pres- sure due to these cosmic rays provides support against gravity for the gas in the ISM. Low-energy (100 MeV) cosmic rays are important for the heating and ioniza- tion of interstellar gas. Unfortunately, their flux is difficult to measure within the heliosphere because of strong modulation by the solar wind. The measured cosmic-ray flux near the Earth is shown in Fig. 1.11 together with a fit based upon a model for the cosmic-ray injection spectrum and energy-dependence of the residence time in the interstellar medium. It is obvious that the correction is substantial at low energies. While the interstellar cosmic-ray flux is diffi- cult to measure directly, the resulting ionization drives the build-up of simple molecules (e.g., OH), which can be studied (Section 8.7). These indirect measure- ments imply a primary cosmic-ray ionization rate in the ISM of CR 2×10−16 (H atom)−1 s−1. Localized regions of much higher ionization rate may occur near associations of massive stars. 1.3.4 Kinetic energy of the ISM Winds from early-type stars and supernovae explosions supply bulk kinetic energy to the ISM. Compared to the stellar radiative budget, the total mechanical energy output is only small, ∼0.5 % (Table 1.2). However, the turbulent energy of the HI is about 6 × 1051 erg kpc−2 in the solar neighborhood and provides support for the HI gas against gravity. The HI also shows ordered vertical flow, at some 5 km s−1 in the Milky Way, as well as the infall of large gas complexes at higher latitudes. The expanding shells blown by individual stars and the superbubbles blown by the concerted action of OB associations have an important influence on the morphology of the ISM. They sweep up and compress the surrounding ISM
1.3 Energy sources 17 KINETIC ENERGY (MeV nucleon–1 ) PROTONS CARBON IRON a a a a b b b b (b) (a) 103 102 101 100 10–1 10–2 10–3 10–4 10–5 10–6 100 101 102 103 104 105 100 101 102 103 104 105 10–7 10–8 103 102 101 100 10–1 10–2 10–3 10–4 10–5 10–6 10–7 10–8 ALPHAS KINETIC ENERGY (MeV nucleon–1 ) DIFFERENTIAL FLUX (nucleon m –2 sr –1 s –1 MeV –1 ) a : q = 4.20, µ = 0.50 b : q = 4.10, µ = 0.60 Figure 1.11 The cosmic-ray proton flux as a function of energy measured near the Earth and the inferred interstellar cosmic-ray flux after the effects of modu- lation by the solar wind have been taken into account. Figure reproduced with permission from W.-H. Ip and W. I. Axford, 1985, A. A., 149, p. 71. and set it into motion. These motions are often unstable to Rayleigh–Taylor and Kelvin–Helmholtz instabilities and, in general, turbulence is expected to take over. This kinetic energy decays through shock waves when clouds collide, emerging as line radiation, or through the excitation of plasma waves, which, eventually, also heat the gas. The average heating due to turbulence is, however, small (Table 1.2). On a smaller scale, individual molecular clouds show linewidths in excess of the thermal width because of the presence of turbulent motions. This turbulence, which is probably of a magneto-hydrodynamic nature, supports these clouds against self-gravity. This turbulent energy is supplied by powerful outflows driven by newly formed stars into their surroundings – and hence derives from the
18 The galactic ecosystem gravitational energy of the collapsing cloud core – or by direct tapping of the magnetic or rotational energy supporting the cloud. 1.3.5 Summary While these energy sources are very similar in magnitude (cf. Table 1.2), how much each of them contributes to the heating of the gas (and dust) depends on the coupling processes. Understanding these processes will thus be of prime importance for understanding the physical state of the ISM. 1.4 The Milky Way Table 1.3 summarizes several characteristics of the Milky Way as a galaxy. Comparing these with those of other galaxies, the Hubble type of the Milky Way seems to be somewhere between an Sb and an Sc. In many ways, the Milky Way seems to be an average spiral galaxy. 1.4.1 Galactic distribution The molecular gas shows an exponential distribution, peaking at the molecular ring (4.5 kpc) with a scale length of about 3 kpc. In contrast, the atomic gas has a rather flat distribution out to about 18 kpc (cf. Fig. 1.12). So, within the solar circle, the surface density of the molecular gas is somewhat larger than that of the atomic gas. In contrast, the atomic gas dominates the molecular gas by far in the outer Galaxy. Within 4 kpc, there is a hole in both the atomic and molecular gas distribution except for the nuclear ring in the inner Galaxy. The total HI mass is about 5 times that of H2. The most striking aspect of the gas and dust distribution of any galaxy is the thinness of the disk. The molecular gas has a thickness of 75 pc compared with a radial scale length of 4 kpc. In the inner Milky Way, the HI scale height is two to three times larger, depending on which neutral gas component is considered. In the outer Galaxy, the neutral gas disk flares to a thickness of about 1 kpc. Nevertheless, this is still small compared to the radial scale length. The gas disk also shows a warp in the outer Galaxy. 1.4.2 Spiral structure It is difficult to discern the spiral structure of our Galaxy from the atomic gas distribution because of the presence of non-circular velocity components. Mol- ecular clouds are better tracers; not least because molecular clouds are more
1.4 The Milky Way 19 Table 1.3 The Milky Way Object Mass (M) Stars 1.8 (11) Gas 4.5 (9) Source Luminosity (L) Stellar luminosities All stars 4.0 (10) OBA 8.0 (9) Gas and dust [CII] 158 m 5.0 (7) FIR 1.7 (10) Radio 1.5 (8) -rays 3.0 (5) Mechanical luminosities SN 2.0 (8) WR 2.0 (7) OBA 1.0 (7) AGB 1.0 (4) flat rotation curve KBH rotation curve Digel R (kpc) 0 0 2 2 4 4 6 6 8 8 10 12 14 16 18 20 HI H2 Wouterloot Mass surface density (M pc –2 ) Figure 1.12 The mass surface density of HI and H2 as a function of galacto- centric radius. The distribution of HI in the outer Galaxy is quite sensitive to the adopted galactic rotation curve. Note that the nuclear ring is not shown. Figure reproduced with permission from T. M. Dame, 1993, in Back to the Galaxy, ed. S. S. Holt and F. Verter (New York: AIP), p. 267.
20 The galactic ecosystem limited to the arms. Long structures are present in either tracer and, for example, the Sagittarius–Carina arm is readily recognized. Nevertheless, whether the Milky Way is a two-arm or four-arm spiral has not been settled. 1.4.3 Spiral galaxies The overall distribution of the ISM can be particularly well studied in other nearby galaxies such as M31. This shows that the bright HII regions, the atomic and molecular gas, and the dust are concentrated in spiral arms. In contrast the interarm regions are much less obscured, although diffuse HI is present throughout the disk as well. Edge-on galaxies such as NGC 891 – an Sb galaxy very similar to our own – show most clearly the narrow disk component; for example, in the dust absorbing background stellar light. The HI distribution of this galaxy shows a component that is somewhat more extended than that of the Milky Way. 1.5 The mass budget of the ISM Stars of all masses in various stages of their lives indiscriminately pollute their environment with gas, dust, and metals. Except for a few elements produced in the Big Bang, the abundance of all heavy elements reflects this enrichment with stellar nucleosynthetic products. Table 1.4 summarizes gas and dust mass injection rates into the ISM for a variety of stellar objects. The dust budget has been split out into two separate columns according to whether the stellar source contains carbon-rich zones (C/O 1), which lead to carbonaceous dust formation, or oxygen-rich zones, which lead to the formation of oxides (silicates) or metals. The quoted values are uncertain – some more than others – and are often based upon various assumptions. Even estimates of the relative importance of high-mass stars versus low-mass stars evolve over time because of new developments in, for example, the determination of mass loss rates (e.g., IR studies) or nucleosynthetic reaction rates (e.g., the 12C()16O rate). Thus, while 20 years ago high-mass stars (M 8 M) were thought to be mainly responsible for the carbon in the interstellar medium, in more recent studies carbon-rich red giants dominate. These injection rates also vary across the Galaxy; i.e., because of the general increase in metallicity towards the inner Galaxy, the ratio of the O-rich to C-rich giants increases towards the galactic center, as does the Wolf–Rayet star population. The gas mass return rate is dominated by low-mass red giants, as expected, since low-mass stars dominate the stellar mass of the Galaxy. However, massive stars are more efficient in producing and injecting heavy elements, such as O and Si, in the ISM (a typical type-II supernova has an enrichment factor of ∼10 for such elements). However, on the AGB, low-mass stars inject much C formed
1.5 The mass budget of the ISM 21 Table 1.4 Interstellar gas and dust budgets Source Ṁa H (M kpc−2 Myear−1 ) Ṁb c (M kpc−2 Myear−1 ) Ṁc sil (M kpc−2 Myear−1 ) C-rich giants 750 30 — O-rich giants 750 — 50 Novae 6 03 003 SN type Ia — 03d 2d OB stars 30 — — Red supergiants 20 — 02 Wolf–Rayet 100e 006f — SN type II 100 2d 10d Star formation −3000 — — Halo circulationf 7000 Infallg 150 a Total gas mass injection rate. b Carbon dust injection rate. c Silicate and metal dust injection rate. d Fraction and composition of dust formed in SN is presently unknown. These values correspond to upper limits. e Dust injection only by carbon-rich WC 8–10 stars. f Mass exchange between the disk and the halo estimated from HI in non-circular orbits and CIV studies. g Estimated infall of material from the intergalactic medium and satellite galaxies. by the triple- process. Asymptotic giant branch stars are also the site of the production of the s process (formed through slow neutron capture) elements. Type-Ia supernovae, which also have low mass progenitors, inject a considerable amount of Fe into the ISM. The time scale for stars to inject or replenish the local interstellar gas mass (8×106 M kpc−2 ) is 5×109 years. The total dust injection rate corresponds to a dust-to-gas ratio in the ejecta of ∼15%. This is somewhat larger than the average dust-to-gas ratio in the ISM, reflecting the synthesis of heavy elements in type-II supernova. Locally, star formation will convert all molecular gas into stars in only 2×108 years; the average star formation rate is heavily weighted towards the inner molecular ring. The different phases of the ISM exchange material at a rapid rate (3×107 years) and, considering all the available gas, this time scale to convert gas into stars increases to 3 × 109 years and is more comparable to the stellar injection time scale. Both rates are very short compared with the lifetime of the Galaxy (12×1010 years). The disk of the Galaxy also exchanges material with the lower halo. In particular, the concerted action of supernovae set up a galactic fountain and some 5 M is exchanged per year. In terms of the mass flux, this circulation pattern dominates the mass budget. Finally, the Galaxy is still accreting primordial material from its environment. The amount
22 The galactic ecosystem of this accretion is controversial. There is a contribution from satellite galaxies such as the Magellanic Clouds, and the Magellanic Stream represents an accretion of some 150 M kpc−2 Myear−1. There are also some indirect arguments that suggest that the Galaxy may have been accreting about 1 M year−1 of metal-poor primordial gas over much of its lifetime or about an order of magnitude more than the Magellanic Stream accretion. 1.6 The lifecycle of the Galaxy The origin and evolution of galaxies are closely tied to the cyclic processes in which stars eject gas and dust into the ISM, while at the same time gas and dust clouds in the ISM collapse gravitationally to form stars. The ISM is the birthplace of stars, but stars regulate the structure of the gas, and therefore influence the star formation rate. Winds from low-mass stars – and hence, the past star formation rate – control the total mass balance of interstellar gas and contribute substantially to the injection of dust, an important opacity source, and polycyclic aromatic hydrocarbon molecules (PAHs), an important heating agent of interstellar gas. High-mass stars (i.e., the present star formation rate) dominate the mechanical energy injection into the ISM, through stellar winds and supernova explosions, and thus the turbulent pressure that helps support clouds against galactic- and self- gravity. Through the formation of the hot coronal phase, massive stars regulate the thermal pressure as well. Massive stars also control the FUV photon energy budget and the cosmic-ray flux, which are important heating, ionization, and dissociation sources of the interstellar gas, and they are also the source of intermediate-mass elements that play an important role in interstellar dust. Eventually, it is the dust opacity that allows molecule formation and survival. The enhanced cooling by molecules is crucial in the onset of gravitational instability of molecular clouds. Clearly, therefore, there is a complex feedback between star formation and the ISM. And it is this feedback that determines the structure, composition, chemical evolution, and observational characteristics of the interstellar medium in the Milky Way and in other galaxies all the way back to the first stars and galaxies that formed at redshifts, z5. If we want to understand this interaction, we have to understand the fundamental physical processes that link interstellar gas to the mechanical and FUV photon energy inputs from stars. 1.7 Physics and chemistry of the ISM The key point always to keep in mind when studying the interstellar medium is that the ISM is far from being in thermodynamic equilibrium. In thermodynamic equilibrium, a medium is characterized by a single temperature, which describes
1.8 Further reading 23 the velocity distribution, excitation, ionization, and molecular composition of the gas. While the velocity distribution of the gas can generally be well described by a single temperature, the excitation, ionization, and molecular composition are often very different from thermodynamic equilibrium values at this temperature. This reflects the low pressure of the ISM, so that, for example, collisions cannot keep up with the fast radiative decay rates of atomic and molecular levels. Ionization and chemical composition are also kept from their equilibrium values by the presence of ∼100 MeV cosmic-ray particles – clearly a non-Maxwellian component – and a diluted, stellar, EUV-FUV photon field, which is much stronger than a 100 K medium would normally have. Finally, the large-scale velocity field is much influenced by the input of mechanical energy. Whenever a gas is not in local thermodynamic equilibrium, the level popula- tions, degree of ionization, chemical composition, and of course the temperature are set by balancing the rates of the processes involved. Much of the study of the ISM is thus concerned with identifying the various processes that control the ionization and energy balance, setting up the detailed statistical equilibrium equations and solving them for the conditions appropriate for the medium. In the remainder of this book, we will thus first study the physical and chemical processes that are of astrophysical relevance (Chapters 2–4). This is followed by a discussion of the physics and chemistry of two important components of the ISM: dust grains (Chapter 5) and large molecules (Chapter 6). With this in hand, we can examine in-depth HII regions, the phases of the ISM, photodissociation regions, and molecular clouds (Chapters 7–10). In each chapter, the ionization and thermal balance are investigated first, setting up the statistical equilibrium equations and derivingtherelevantparametersofthemedium.Thisisthenfollowedbyadiscussion of the observations of these objects and their analysis. In Chapters 11–12, non- equilibriumeffectsofadifferentnaturearestudied.Specifically,thetime-dependent effects of strong shock waves on the temperature and chemical composition of clouds are discussed in Chapter 11. Chapter 12 examines then the dynami- cal effects of expanding HII regions, supernova explosions and stellar winds. Finally, in Chapter 13, some aspects of the lifecycle of interstellar dust – which is a prime example of these types of non-equilibrium effects – are explored. 1.8 Further reading A general, technical yet accessible, introduction to the interstellar medium is provided by [1]. An introductory-level discussion of the interstellar medium can be found in [2]. A thorough but now dated monograph on physical processes in the interstellar medium is [3], which is an updated version of [4]. These two textbooks are
24 The galactic ecosystem no easy reading matter and the reader should beware that many a promising young scientist was last observed buying these books. A recent monograph on the interstellar medium is [5], which is intended for postgraduate courses. An unsurpassed textbook treating the physics of ionized gas is [6]. A recent overview of the properties of our Galaxy as compared to other galaxies is given by [7]. Panoramic images of the Milky Way at different wavelengths can be found on the web, http://adc.gsfc.nasa.gov/mw/milkyway.html. Each of these wavelengths traces a different component of the Galaxy. It is instructive to compare these different images. References [1] J. B. Kaler, 1997, Cosmic Clouds: Birth, Death, and Recycling in the Galaxy, (New York: Freeman Co.) [2] J. E. Dyson and D. A. Williams, 1980, Physics of the Interstellar Medium (Manchester: Manchester University Press) [3] L. Spitzer, Jr., 1978, Physical Processes in the Interstellar Medium (New York: Wiley) [4] L. Spitzer, Jr., 1968, Diffuse Matter in Space, (New York: Interscience) [5] M. A. Dopita and R. S. Sutherland, 2003, Astrophysics of the Diffuse Universe, (Berlin: Springer Verlag) [6] D. Osterbrock, 1989, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (Mill Valley: University Science Books) [7] R. C. Kennicutt, 2001, in Tetons 4: Galactic Structure, Stars and the Interstellar Medium, ed. C. E. Woodward, M. D. Bicay, and J. M. Shull (San Francisco: ASP), p. 2
2 Gas cooling In this chapter, we will examine the various line cooling processes of interstellar gas. Together with the heating processes, which are discussed in the next chapter, this will allow us to solve the energy balance for the gas in a wide variety of environments. This will be applied in HII regions (Section 7.3), HI regions (Section 8.2.2), photodissociation regions (Section 9.3), and molecular clouds (Section 10.3). This chapter starts off with a refresher on the concepts of electronic, vibrational, and rotational spectroscopy (Section 2.1). We will then discuss the cooling rate (Section 2.2) with the emphasis on two-level systems (Section 2.3). A two-level system analysis is a powerful tool for understanding the details of gas emission processes and we will encounter this many times in subsequent chapters. We will then consider in detail the cooling processes in ionized (Section 2.4) and neutral (Section 2.5) gas. The last section (Section 2.6) discusses the cooling law. 2.1 Spectroscopy Table 2.1 summarizes typical properties of transitions. These are, of course, directly related to the binding energies of the species involved. Electronic binding energies for atoms increase from left to right in the periodic system from about 5 eV to some 20 eV. For hydrogen and helium the lowest electronic transitions are fairly high (10.2 and 21.3 eV, respectively); a substantial fraction of the ionization energy. Multi-electron systems have electronic orbitals that are low in energy compared with their ionization potentials. Their electronic transitions thus occur in the visible through extreme UV range. Transitions of bonding electrons in molecules typically also occur in the UV range. However, radicals or ions – which have an unpaired electron in low-lying electronic states – have transitions in the visible range. Molecular vibrational transitions involve motions of the atoms and, because of the larger reduced mass, are shifted by a factor me/M into the mid-infrared, where me and M are the masses of the electron and atom, 25
26 Gas cooling Table 2.1 Typical transition strengtha Type of transition ful Auls−1 Example Auls−1 Electric dipole UV 1 109 Ly 1216 Å 240×108 Optical 1 107 H 6563 Å 600×106 Vibrational 10−5 102 CO 4.67 m 34.00 Rotational 10−6 3×10−6 CSb 6.1 mm 170×10−6 Forbidden Optical (Electric quadrupole) 10−8 1 [OIII] 4363 Å 1.7 Optical (Magnetic dipole) 2×10−5 2×102 [OIII] 5007 Å 200×10−2 Far-IR fine structure 2×10−7 m 10 3m [OIII] 52 m 980×10−5 Hyperfine HI 21 cm 290×10−15 a See text for details. b The J = 1 → 0 transition. respectively. Molecules can also rotate and the centrifugal energy is proportional to 1/M. Hence, rotational energies of molecules are smaller by a factor me/M compared with electronic energies and occur at (sub)millimeter wavelengths. As an example, the binding energy of CO is 11 eV (1100 Å), the vibrational energy is 0.27 eV (2170 cm−1467m), and the rotational energy is 5×10−4 eV (2.6 mm). A final and important point to keep in mind when considering transitions between levels is that they obey certain selection rules depending on whether they are electric dipole, magnetic dipole, electric quadrupole, etc. This directly influences the strength of the transitions (Table 2.1 and Section 2.1.4). We will come back to this later (Table 2.2 and Section 2.1.4). 2.1.1 Electronic spectroscopy Atoms Each atomic line can be related to a transition between two atomic states. These states are identified by their quantum numbers. The states are characterized by the principal quantum number, n (1, 2, …); the orbital angular momentum, (which can have values between 0 and n−1); and the electron spin angular momentum, s±1/2. The orbital angular momenta are designated by s, p, d, … , corresponding to = 0 1 2, … The total angular momentum, j, is the vector sum of and s. For hydrogen, neglecting fine structure, the energies of the atomic states depend only on n. Hydrogen transitions can be grouped into series (downward transitions, m → n, or upward transitions, n → m) according to the value of n, with the conventional names for the first few: Lyman (n = 1), Balmer (n = 2), Paschen
2.1 Spectroscopy 27 Table 2.2 Selection rulesa Electric dipole Magnetic dipole Electric quadrupole “allowed” “forbidden” “forbidden” 1 J = 0±1 J = 0±1 J = 0±1±2 0 0 0 0 0 0, 1/2 1/2, 0 1 2 M = 0±1 M = 0±1 M = 0±1±2 0 0 when J = 0 0 0 when J = 0 3 Parity change No parity change No parity change 4b One electron jumping For all electrons One electron jumping with l = ±1n arbitrary l = 0n = 0 l = 0±2n arbitrary or for all electrons l = 0n = 0 5c S = 0 S = 0 S = 0 6c L = 0±1 L = 0J = ±1 L = 0±1±2 0 0 0 00 1 a A short summary of six selection rules describing electric dipole and quadrupole and magnetic dipole transitions. b With negligible configuration interaction. c For LS coupling only. Hydrogen energy levels (a) 1 1 s 3 2 p d f –1 –3 –5 –7 –9 –11 –13 E (eV) (b) Hydrogen energy levels 1 1 s 3 2 p d f –1 –3 –5 –7 –9 –11 –13 – 15 E (eV) Figure 2.1 The energy level diagram of hydrogen. The principal quantum number, n, increases upwards (for clarity only the lower ones are labeled). The orbital angular momentum number, , increases rightwards and only the first few are shown. They are labeled s, p, d, f. The arrows indicate some allowed transi- tions. (a) The Lyman through transitions are indicated. (b) The transitions between the lowest levels are indicated. Note that the 2s level is metastable. (n = 3), Brackett (n = 4), Pfund (n = 5), and Humphreys (n = 6). Successive lines in these series are labeled , etc. Thus, Lyman emission corresponds to the transition n = 2 → 1. The Balmer transitions are also indicated by the designation, H, H, etc. For heavier species, the electrons are grouped in closed and open shells. The shell energies depend mainly on n. Subshells within each shell have different
28 Gas cooling angular momenta and are labeled by n, and the number of electrons in the subshell (to the maximum possible 22+1). For heavier species, the description of states involved in transitions becomes more complicated than for H because of the interaction between the various electrons present. The energy levels of an atom are characterized by the values of J, the magnitude of the total angular momentum. The total angular momentum, J is defined by J = L+S. Here, L is the orbital angular momentum: the vector sum of the orbital angular momenta of the individual electrons. The spin of the atom, S, is the sum of the electron spins, where each electron has a spin 1/2 that can be parallel or antiparallel. The total angular momentum, J, is formed by vector addition and can take the integral values, L+SL+S−1L+S−2L−S. There are thus 2S+1 possible J values. When relativistic effects are small, a level with a given L and S is split into a number of distinct levels with different values of J, because of spin-orbit coupling. This coupling is a result of the interaction of the magnetic moment of the electron spin with the magnetic moment of the orbital motion. This fine-structure splitting is a small perturbation – of the order of 2 Z4 , with the fine-structure constant = 1/137 and Z the nuclear charge. Transitions among the fine-structure levels occur therefore at near-to far-IR wavelengths. Energy levels are then designated by term symbols, 2S+1 LJ , with the different values for the total orbital angular momentum, L = 012 , denoted by capital roman letters, S, P, D, …1 The multiplicity of the term, 2S +1, can take values 1, 2, 3, … , and leads to the designation singlet, doublet, triplet, … , levels. Each of these levels is still degenerate with respect to the orientation of J; e.g., J is space quantized in a magnetic field. Designating M as the component of J along the direction of the magnetic field, M can take the values, JJ −1J −2−J (e.g., gJ = 2J +1). Generally, the nuclear spin is of little concern. However, for HI, the hyperfine structure, resulting from the interaction of the magnetic moments of the electron and the nucleus, is of particular interest because it gives rise to the 21 cm line. This interaction is quite weak and the resulting splitting is very small. All this terminology of electronic terms can be interpreted in classical terms of orbital and spin angular momentum and the coupling between them. However, in addition, there is the quantum-mechanical concept of parity, which has no classical analog. Parity describes the behavior of the wave function, xyz, under reflection where even parity corresponds to xyz = −x−y−z and odd parity, xyz = −−x−y−z. Odd parity is indicated by a left superscript o. Selection rules for electronic transitions are summarized in Table 2.2. These merely reflect conservation principles. Thus, angular momentum has to be 1 Unfortunately, convention uses very similar notation for L = 0 (S) and for the electron spin angular momen- tum (S).
2.1 Spectroscopy 29 conserved under vector addition and, given that the photon has one unit of angular momentum, this leads to the J and L rules. The electron spin can only be changed by a magnetic field. The parity rule reflects that the dipole operator in the transition moment integral (proportional to the transition probability) has odd parity and hence only couples states with opposing parity. In the weak spin- orbit coupling regime (LS-coupling, also called Russell–Saunders coupling), the fine-structure energies are small compared to the energy differences of the levels. In the LS-coupling case, both L and S are “good” quantum numbers, which are conserved. When the coupling increases, (e.g., for heavier elements with larger nuclear charge), this rule breaks down and the spin-orbit interactions become as strong as the interactions between individual spins or orbital angular momenta. For LS coupling, conservation of S results in the segregation of transitions into groups associated with singlet, triplet, or quintet states, and transitions between these different S-states are weak. Conversely, this then implies that a species can become “trapped” in the lowest triplet or quintet state, which can represent considerable internal excitation. As an example, consider neutral carbon with six electrons (1s2 2s2 2p2 ). In the ground state, four of these are paired up in 1s and 2s orbitals and two occupy 2p orbitals but with the same spin (e.g., 1s22s22p2 where the superscript indicates the number of electrons in the shell). These parallel spins give favorable exchange interactions. The ground state has thus L = 1S = 1, and J = 0, e.g., 3P0. Fine- structure interaction then produces two levels, 3P1 and 3P2 at 16.4 and 43.4 cm−1 above the ground state. The state where the electrons have opposing spin has slightly higher energy and has L = 2S = 0, and J = 2 with designation, 1 D2 at about 10 000 cm−1 above ground. The next state up is a 1 S0 state at about 21 600 cm−1 . These levels arise from the same electron configuration and are thus only linked through forbidden transitions. In particular, the 3 P levels are connected through fine-structure transitions at 609 and 370 m. The forbidden character of transitions between the low-lying states is a very general point: all the low-lying electronic levels of the more common species arise from the same electronic configuration as their ground state and transitions are forbidden because of the parity selection rule. Molecules We will first discuss diatomic molecules, which in many ways we can describe in terms of an equivalent atom. Many of the concepts of atomic spectroscopy carry directly over into molecular spectroscopy. The nuclei in diatomic molecules produce an electric field along the internuclear axis. The component of the angular momentum vector of each electron along the internuclear axis can only take on values ml = ll − 1l − 2−l. The variable, , is then defined
30 Gas cooling as the absolute value of ml, where = 012 correspond to completely analogous to atomic s, p, d, orbitals. For centrosymmetric species (e.g., homonuclear diatomics), parity under inversion is indicated by g for even and u for odd (the wave function does not change sign and changes sign, respec- tively). The term symbols of electronic levels in diatomic molecules are then characterized by the component of the total angular momentum along the inter- molecular axis, equal to the sum of the s. Analogous to the designation, S, P, D, for atoms, we have for diatomic molecules, corre- sponding to = 012 The multiplicity of the term is indicated by a super- script with the value 2S + 1, reflecting the orientation of the total spin angular momentum, , with respect to the internuclear axis, where can take the values SS − 1S − 2−S.2 The total angular momentum is the sum of the orbital and spin angular momenta along the internuclear axis, = + . We can then designate molecular terms as 2S+1ug, where the subscripts only apply to centrosymmetric species. For terms, a ± superscript indicates the behavior of the wave function under reflection in the plane containing the two nuclei. As for atoms, coupling of the magnetic moments associated with spinning/orbiting charges is important. In this case, in addition to the orbital and spin angular momenta, we also need to consider the nuclear rotations. Two limiting cases are of interest: Hund’s case (a), where the latter is unimportant and spin–orbit coupling is dominant, and Hund’s case (b), where the spin couples strongly to the nuclear rotations and weakly to the orbital motions. As for atoms, the total angular momentum must change by 0 or 1 (and J = 0 0). The selection rules for allowed electronic transitions are now = 0±1S = 0 = 0, and = 0±1. The selection rules associ- ated with symmetry are now: for terms, only + ↔ + and − ↔ − are allowed. For centrosymmetric species, only transitions with a change in parity (e.g., u ↔ g) are allowed. As for atoms, the conservation of electronic spin can lead to metastable excited levels. So, molecular hydrogen has the ground state 1 + g with S = 0, = 0, and = 0 and bound excited singlet states, 1 + u and 1 u (Fig. 2.2). There is also a repulsive electronic triplet state, 3 + u , at intermediate energies and bound excited triplet states (3u and 3 + g ). Molecular oxygen has a ground state 3 − g and carbon monoxide has the ground state 1 +. Singlet–triplet transitions are forbidden and the first allowed electronic transitions for molecular hydrogen are the Lyman (1 + g →1 + u ) and Werner (1 + g →1 u) bands. In polyatomic molecules, electronic transitions are often connected to the exci- tation of specific types of electrons or electrons associated with a small group of 2 Again, convention uses very similar notations for the = 0 term () and the total spin angular momentum ().
2.1 Spectroscopy 31 H(1s)+p+e 145 792 H(1s)+H(2s,2p) 118 372 H(1s)+H(1s) 36 113 cm–1 cm–1 124 429 1.06 99 143 90 203 1.03 1.29 14.671 18.071 v = 14 v = 10 v = 5 eV 4.476 v = 0 0.7416 –2179 0 1 2 Å 800 Å 850 Å 100 Å 1108 Å H+ 2 1Σ+ u 1 Σ+ g 2 Σ+ g 3Σ+ g 1 Πu 3Πu 3Σu + 1 Σg + Figure 2.2 Schematic of the low-lying electronic energy levels of H2 as a function of the separation between the two hydrogen atoms. Some vibrational levels in the ground electronic state are also schematically included. Dissoci- ation and ionization energies are indicated on the right-hand side in eV and cm−1 . Some excitation energies are given on the left-hand side in cm−1 . All energies are relative to the v = 0 level in the ground electronic state. Figure reproduced from G. B. Field, W. B. Somerville, and K. Dressler, and reprinted with permission from Ann. Rev. Astron. Astrophys., 4, p. 207, ©1966, by Ann. Rev. (www.annualreviews.org). atoms in the species. For example, the ← transition in a C= C bond occurs around 60 000 cm−1 (1600 Å), while in a C= O bond this transition occurs around 36 000 cm−1 (2800 Å). In a conjugated chain, this transition will shift to longer wavelengths. It is not possible to review the spectroscopy of polyatomic species here. In short, for a given nuclear geometry, molecular orbitals are constructed from atomic orbitals. The available electrons are then divided over these orbitals to construct electronic configurations, taking electron correlation into account (e.g., the Pauli principle). The lowest electronic configuration is then the ground state. For organic molecules, the ground state possesses a “closed shell” configu- ration (i.e., all the bonding and non-bonding orbitals are filled with a pair of elec- trons). Radicals (and often ions) possess molecular orbitals with only one electron. For our purposes, the relevant electrons are located in the highest filled molec- ular orbitals. Deeper-lying electrons are not easily excited by the relatively low energies of photons in the interstellar medium. Radicals have transitions at low energies (i.e., in the visible range of the spectrum) associated with the partially occupied molecular orbital. Ground-state configurations for species where the
32 Gas cooling electrons are paired must be singlet and are labeled S0 where the S indicates singlet and the 0 the ground state. Electronically excited singlet states are then enumerated according to their energy as S1S2 Excited states do not have to have paired electron spins. Hence, we can distinguish triplet states, which are designated T1T2 (the subscript 0 is exclusively assigned to the ground state). The lowest triplet state is at somewhat lower energy than the first excited singlet state, reflecting the better electron correlation in the former. This energy differ- ence ranges from ∼03 to 3 eV. For polycyclic aromatic hydrocarbons this energy difference is typically 1–1.5 eV (Fig. 2.3). As for atoms and diatomic molecules, allowed radiative transitions have no change in electron spin. Hence, the lowest triplet state is metastable with respect to the ground state. Up to now, we have purposely avoided the most striking aspect of molecular spectra, the presence of considerable substructure associated with nuclear vibra- tions and rotations. The quantization of vibrations and rotations will be discussed in more detail in Sections 2.1.2 and 2.1.3. What is of importance here is that during Figure 2.3 Schematic diagram of singlet and triplet electronic levels of an organic molecule. The electronic states are displaced horizontally for clarity but this has no physical meaning. The vertical axis denotes energy on an arbitrary scale. Within each electronic state, vibrational levels are schematically displayed. The rotational sublevels are not shown.
2.1 Spectroscopy 33 an electronic transition, the molecule may also change its vibrational or rotational state. This may result in vibrational sidebands, which are shifted in energy by one or more quanta of vibrational energy (∼1000–3000 cm−1 ). In addition, these bands also show small-scale structure caused by simultaneous rotational transi- tions. As for vibrations, these are classified in terms of classical P, Q, and R branches (cf. Section 2.1.2). The general appearance of these bands reflects the difference in rotational constants between the two electronic states involved in the transition. 2.1.2 Vibrational spectroscopy The essence of vibrational spectroscopy is contained within Hooke’s law for a harmonic oscillator, = 1 2 c (2.1) with the fundamental frequency, the force constant, and the reduced mass of the molecular units vibrating. The factor c is included to transform the unit to wavenumbers (cm−1) for cgs units, commonly used in spectroscopy. Molecular bond strengths are, of course, very similar to binding energies of electrons to an atom. However, the frequencies of the transitions of molecular vibrational levels are, thus, shifted to lower energies by me/M, with me and M the masses of the electron and atom, respectively, and occur in the near- and mid-IR. Real molecules are not harmonic oscillators and, as a result of anharmonicity of the bonding, hot bands (e.g., 2–1, 3–2) are generally shifted to lower frequencies. All the possible vibrational motions of a molecule can now be described in terms of these fundamental modes. A non-linear molecule containing N atoms will have 3N − 6 normal modes; a linear molecule 3N − 5. Not all of these modes will lead to distinct absorptions. Some of the modes will occur at the same frequency and hence these modes are degenerate. Others will not be infrared-active because the dipole moment does not change during the vibration. So, as an example, methane has nine fundamental modes but only the symmetric and asymmetric stretching and bending vibrations of the C–H bonds about the central C atom show up in the IR absorption spectrum: the 41306cm−1 , the 21534cm−1 , and the 33019cm−1 modes. Vibrations in homonuclear molecules do not lead to changes in the dipole moment and hence are inactive in the infrared. Mixing of isotopes in such species will lead to infrared absorptions. Likewise, interactions with the environment in a solid will introduce weak infrared activity. Vibrational transitions are very characteristic for the motions of the atoms in the molecular group directly involved but much less sensitive to the structure of the rest of the molecule. Characteristic band positions of various molecular groups
34 Gas cooling X–H bond stretching modes C C C C C C C C N O O O O O O O O C C C C C C H H H H H H H H H H O C C C C C C C C S N H H O N 4000 cm–1 3500 3000 2500 2000 1500 1000 500 20 15 10 9 8 7 6 5 4 Wavelength (µm) 3 2.5 µm H H H X–X bond stretching modes bond bending modes C Figure 2.4 Summary of the vibrational frequencies of various molecular groups. The filled boxes indicate the range over which specific molecular groups absorb. The vibrations of these groups are schematically indicated in the linked boxes. Figure kindly provided by D. Hudgins. are illustrated in Fig. 2.4 and summarized in Table 2.3. Modes involving motions of hydrogen occur at considerably higher frequencies than modes involving similar motions of heavier atoms (Hooke’s law again). Thus, H-stretching vibrations occur in the 3 m region, while stretching motions among (single-bonded) C, N, and O atoms are located around 10 m. Likewise, when the bond strength increases, the vibration shifts to higher frequencies. Simplistically speaking, single bonds are characterized by two electrons occupying the bonding molecular orbital. Similarly, double bonds and triple bonds correspond to four and six electrons in the bonding molecular orbitals. Because the bond strength increases from single bonds to double bonds to triple bonds, the location of singly, doubly, and triply bonded CC vibrations shifts from about 1000 to 2000 cm−1 (Fig. 2.4). The actual spectrum of a gaseous compound is much more complex because transitions are actually combined rotational–vibrational transitions where both the vibrational and the rotational energy can change. This gives rise to three sets of absorption bands corresponding to J = 1 (R branch), J = 0 (Q branch), and J = −1 (P branch), where J = J −J with J and J the rotational quan- tum numbers in the upper and lower vibrational state, respectively. For elec- tric quadrupole transitions, the selection rules are J = 2 (S branch), J = 0 (Q branch), and J = −2 (O branch). These sets of bands are, of course, shifted from each other by the difference in rotational energy content of the species. Consider as an example a diatomic molecule, which we represent here as a harmonic oscillator and a rigid rotor. The energy is then given by EvJ = v+ 1 2 +B J J +1 (2.2) with v and J the vibrational and rotational quantum number, the vibrational frequency, and B the rotational constant. The R branch (v = 1 → 2 J → J + 1)
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calm and exalted. Her blood ran light. Having destroyed her world, her disbelief somehow survived as if on an eminence. However, her emotions rejected their own finality. She felt that she had to go on somewhere outside herself. May waited in vain for Paul to come back. She convinced herself that she was not good. When she believed in her own humility she was not afraid to admit that she wanted to see him. She was unhappy now with her own body. As soon as she saw her little breasts uncovered she felt frightened and ashamed and wanted to hide herself. When she was alone in her room she cried miserably, but as soon as her tears ceased to flow she lay on her bed in an empty waiting happiness, thinking of Paul. She recalled all that related to him since she had first known him. It gave her a beautiful happy sense of want to remember him so distinctly. However, when her thoughts arrived at the memory of the last thing that had occurred between them she imagined that she wished him to kill her so that she need no longer be ashamed. I want to be dead! I want to be dead! She said this over and over into her pillow. Her beautiful pale braid of hair was in disorder. Her thin legs protruded from her wrinkled skirts. She lifted her small tear-smudged face with her eyes tight shut. May wanted to tell Aunt Julia, but dared not. She knew Aunt Julia was sad, though she did not know why. Aunt Julia, however, resisted confidences. When she came in from work and found May waiting for her in the hall or on the stairs Aunt Julia made herself look tired and kind. Well, May, dear, how are you? You seem to be a very bored young lady these days. Your father is thinking of sending you away to school when Bobby goes. How would you like that? And she smiled in a perfunctory far-away fashion.
May saw that Aunt Julia was in another world and did not want her. I don't care. Whatever you and Papa decide. I'm an awful ninny and should be terribly homesick. That would be good for you. You must learn to be self-reliant. Without glancing behind her, Aunt Julia passed quickly up the stairs and disappeared into her room. The door shut. To May it was as if Aunt Julia knew everything already and put her aside because of what she had done. She was dead and corroded with shame. Lonely, she wandered out into the back yard. The sky, in the late sunshine, was covered with a pale haze like faint blue dust. A shining wind blew May's hair about her face and swirled the long stems of uncut grass. The seeded tops were like brown-violet feathers. Beyond the roofs and fences the horizon towered, vast and cold looking. May wanted it to be night so that she could hide herself. She knew Nellie was in the kitchen doorway watching her. She wanted to avoid the eyes of the old woman. Paul could not love her while she was despised. White clothes on a line were stretched between the windows of the apartment houses that overhung the alley. The bleached garments, soaked with blue shadow, made a thick flapping sound as the wind jerked them about. When the sun sank the grass was an ache of green in the empty twilight. May thought it was like a painful dream coming out of the earth. She was afraid of the fixity of the white sky that stared at her like a madness. She knew herself small and ugly when she wanted to feel beautiful. If she were only like Aunt Julia she would not be ashamed. It grew dark. She loved the dark. There was a black glow through the branches of the elm tree against the fence. The large stars, unfolding like flowers, were warm and strange. In the enormous evening only a little shiver of self-awareness was left to her. She tried to imagine that, because she was ugly and impure, Paul had
already killed her. The strangeness and exaltation she felt came to her because she was dead. She loved him for destroying her. Dudley gave up the attempt to take Laurence into his life. Dudley had insisted on seeing the Farleys several times, but the result of these meetings was always disappointing. What he considered their small hard pride erected about them a wall of impenetrable reserves. He pitied them in their conventionality. They regard me, he thought, as a wrecker of homes, and the fact that I have been Julia's lover prevents them from recognizing me in any other guise. He felt that he was learning a lesson. He must avoid destructive intimacies. If he gave, even to small souls, he had to give everything. In order to save himself for his art he must learn to refuse. He was in terror of love, in terror of his own necessities, and afraid of meeting acquaintances who, with the brutality of casual minds, could shake his confidence in himself by uncomprehending statements regarding his work. He grew morbid, shut himself up in his studio, and refused to admit any validity in the art of painters of his own generation. He persuaded himself that he was the successor of El Greco and that since El Greco no painter had done anything which could be considered of significance to the human race. He would not even admit that Cézanne (whom he had formerly admired) was a man of the first order. He was a painter, to be sure, but Dudley could ally himself only with those whose gifts were prophetic. His imaginings about himself assumed such grandiose proportions that he scarcely dared to believe in them. To avoid any responsibility for his conception of himself he was persuaded that there was a taint of madness in him. Rather than awaken from a dream and find everything a delusion, he would take his own life. He lay all day in his room and kept the blinds drawn, and was tortured with pessimistic thoughts, until, by the very blankness of his misery, he
was able to overcome the critical conclusions of his intelligence. He did not eat enough and his health began to suffer. His absorption in death drew him to concrete visions of what would follow his suicide. He was unable to close his eyes without confronting the vision of his own putrid disintegrating flesh. In his body he found infinite pathos. As much as he wanted to escape his physical self, it was sickening to think of leaving it to the indignities of burial at the hands of its enemies. The idea of suicide, haunting him persistently, aroused a resistant spirit in him. He exaggerated the envies of his contemporaries. He fancied that they feared him far more than they actually did and were longing for his annihilation. He decided that something occult which originated outside him was impelling him toward self- destruction. In refusing to kill himself he was combating evil suggestions rather than succumbing to his own repugnance to suffering and ugliness. While he was in this frame of mind some one sent him a German paper that was the organ of an obscure artistic group. In this journal, insignificantly printed, was a flattering reference to Dudley. He was called one of the leaders of a new movement in America. He read the article twice and was ashamed of the elation it afforded him. He could not admit his deep satisfaction in such a remote triumph. With a sense of release, he indulged to the full the vindictiveness of his emotions toward his own countrymen—those who were fond of dismissing him as merely one of the younger painters of misguided promise. However, the praise from men as unrecognized as himself encouraged his defiance to such a point that he resumed work on a canvas which he had thrown aside. His own efforts intoxicated him. He refused to doubt himself. Life once more had the inevitability of sleep. He knew that he was living in a dream and only asked that he should not be disturbed. He needed to run away from the suggestion of familiar things. He decided to go abroad again and wrote to borrow money of his father.
Dudley made up his mind to avoid Paris where, as he expressed it, the professional artist was rampant. He wanted to visit the birthplace of a Huguenot ancestor who had suffered martyrdom for his religion. It stimulated him to think of himself as the last of a line whose representatives had, from time to time, been crucified for their beliefs. Two endless streams of people moved, particolored, in opposite directions along the narrow street. The high stone buildings were tinged with the red of the low sunshine. Hundreds of windows, far up, catching the glare, twinkled with the harsh fixity of gorgon's eyes. Beyond everything floated the pale brilliant September sky overcast by the broad rays which stretched upward from the invisible sun. Julia, returning from the laboratory, hesitated at a crowded corner and found Dudley beside her. This is pleasant, Julia. I've been wanting to see you and Laurence Farley. I'm sailing for Europe next week, and I should have been very much disappointed if I had been obliged to go off without meeting you again. He tried to speak easily while he looked at her with an expression of reproach. Julia smiled and held out her hand. There was a defensive light in her eyes which he interpreted as a symptom of dislike. He wanted to convince himself that every one, even she, was completely alienated from him. All that fed his pain strengthened his vacillating egotism. Julia noted the familiar details of his appearance: his short arms in the sleeves of a perfectly fitting coat; the plump hairy white hand which reached to hers a trifle unsteadily; his short well-made little body that he held absurdly erect; the wide felt hat that he tried to wear carelessly, which, in consequence, was slightly to one side on the back of his head and showed his dark curls; the childishly fresh color which glowed through the beard in his carefully shaven cheeks;
his small full mouth that sulked in repose but when he smiled displayed exaggeratedly all of his little even teeth; his prettily modeled, womanish nose; the silky reddish mustache on his short lip; and his soft, ingratiating, long-lashed eyes. Everything in his appearance disarmed her resentment of him. Yet she knew that if she expressed anything of her state of mind he would take advantage of her vulnerability. She was prepared to see his gaze harden toward her and his demeanor, puerile now, become ruthless and commanding. She could not analyze the thing in herself that made her so helpless before him. She was able, she thought, to observe him coldly. She withdrew her hand from his and said, So you are going away again? I am glad for your sake. I know how America must irk you. Even from my viewpoint I can see that it is the last country for an artist. At the same moment her heart contracted and she told herself that there was something false and monstrous in Dudley which suppressed her natural impulse to be frank in stating what she felt for him. Dudley walked beside her. She wants me to go away! He insisted on believing this. To know that she continued to suffer, however, comforted him as much now as it had in the past. He sensed that she had, in some remote way, remained subject to him. Because of this she was dear. When he remembered that, but for this accidental meeting, he would not have communicated his departure to her he was momentarily panic-stricken. He no longer wished to detach himself from her. Tell me about your work. What are you doing now? He took her arm. I can't talk about my work, Julia. Something goes out of me that ought to go into the work when I talk about it too much. That's my struggle—my fight. It's terrifying at times. I know all the hounds are baying at my heels. When I go abroad this time I am going to avoid Paris. I know dozens of cities. Paris is the only one which is a work of art. That's why I am going to keep away. I am through with the finality of that kind of art. I am going abroad to feel how much of an American I am. That's why I hate it so. It's in
me—a part of me. I can't escape it. I must express it. That is my salvation—in belonging to America. It was almost irresistible to tell her some of the conclusions he had arrived at to comfort himself, but he knew that Julia never approached a subject from a cosmic angle. She made him feel small and unhappy and full of a homesickness for understanding. In her very crudity she was the life he had to face. I want to talk to you about yourself, Julia. There are clouds of misunderstanding between us. We mustn't leave things like this. He pressed her arm against his side. She was ashamed before a stout woman who was passing who showed, by the expression of dull attention in her eyes, that she had overheard his remark. In this atmosphere of public intimacy Julia felt grotesque. I can't talk about myself, Dudley. Don't ask me. You've put me out of your life. Why should you be interested? He was conscious of the stiffening of her body as she walked beside him and observed the forced immobility of her face. Emerging from the self-loathing which was an undercurrent to his vanity, he was grateful to her for allowing him to hurt her. He began to wonder if he were not, at this instant, realizing for the first time the significance of his relationship to her—not its significance in her life, but its significance in his own. He admitted to himself the cruelty of his feeling for her. He wanted to torture her, to annihilate her even. It pleased him to discover in himself enormous capacities for all things that, to the timid-minded, constitute sin. He must embrace life without moral limitations. Julia, my dear—you must not misunderstand my feeling for you. I want you—want you even physically—as much as I ever did. His voice shook a little. It is only because I understand now that I must refuse myself much. I have found just this last month a marvelous spiritual rest which makes living deeply more acceptable. Julia had never felt more contemptuous of him. What I have to say would only convince you of my limitations. Don't be childish, Julia. You don't want to understand me. We can't talk in the street. Come to my studio for half an hour. He could not
let her go away from him yet. Julia's pride would not allow her to object. On the way they passed an acquaintance of Dudley's. Dudley could not explain to himself why he was ashamed of being seen with Julia. He wanted to hurry her through the street. In the oncoming twilight the brilliant shop fronts were vague with glitter and color. Above the glowering tower of an office building a blanched star twinkled among faded clouds. When they reached Dudley's doorstep Julia began to feel morally ill and to wonder why she had come. As Dudley watched her mount the long green- carpeted stairs before him he was suddenly afraid of her. They entered the studio. It was almost dark in the big room. The canvas that Dudley was working on stood out conspicuously in the translucent gloom that filtered through the skylight. He crossed the floor and furtively threw an old dressing gown over the painting. Julia found herself unable to speak. When she discerned the lounge she sat down weakly upon it. Dudley stumbled over the furniture. He wanted to evade the moment when he must find the lamp. Take off your wrap, Julia. I can't find matches. I seem to have mislaid everything. I am a graceless host. His own voice sounded strange to him. When at last he struck a match, Julia said, Don't! and put her hands to her eyes. The flame, which, for an instant, had blindly illumined his face, went out. Dudley could not bring himself to move. The evening sky, dim with color, was visible through the windows behind him, and above the sombre roof of the factory that rose from the courtyard his figure was thrown into relief. Objects over which there seemed to brood a peculiar stillness loomed about the room. The tension was intolerable to them both. They were experiencing the same nausea and disgust of their emotions—emotions which seemed inevitable for such a moment and so meaningless. Dudley said, Where are you? I'm afraid of stumbling over you.
Julia, a hysterical note in her voice, answered, Here I am, Dudley. She knew that he was coming toward her. She wanted to die to escape the thing in herself which would yield to him. But at this instant the light flashed on and everything that she was feeling appeared to her as unjustifiable and ridiculous. To Dudley, Julia's body represented all the darkness of self-distrust and the coldness of his own worldly mind. He wished that her personality were more bizarre so that he might regard his past acts as mad rather than commonplace. He did not know why he had brought her to the studio and was ashamed to look at her. There was nothing for it but to admit the duality of his nature, and that half of it was weak. He longed to hasten the time of sailing when he would begin completely his life alone in which nothing but the artist in him would be permitted to survive. He said, Is it too late for me to make you some tea? Let me take your wrap. When he approached her he averted his gaze. I can't stay long, Dudley. It is better that I shouldn't. She wanted to force on him an admission of her defeat. If she could only reproach him by showing him the destruction of her self-respect! Her eyes were purposely open to him. He would not see her. She resented his obliviousness. You seem to me a master of evasion. When he sat down near her, he said, Let it suffice, Julia, that I take the hard things you want to say to me as coming from a human being whom I respect and care for enormously—and I still think everything fine possible between us provided you accept in me what I have never doubted in you—my absolute good faith, and my absolute desire, to the best of my powers, to be honest and sincere in every moment of our relationship, past and present. Julia gave him a long look which he obliged himself to meet. Then she got up. I can't stay, Dudley. You won't understand. She turned her head aside. Her voice trembled. It's painful to me. He rose also, helplessly. He wanted to wring a last response from her. It was impossible. Everything seemed dark. He would not
forgive her for going away. Julia took up her wrap from a chair and went out hastily without looking back. Dudley felt a swift pang of despair. Not because she was gone, but because her going left him again with the problem of reviving the hallucinations of greatness. It was not easy for him to deceive himself. He could do so only in the throes of emotions which exhausted him. In moments of unusual detachment he perceived the faults in himself as apart from the real elements of genius that existed in his work. But he was not strong enough to continue his efforts for the sake of an imperfect loveliness. Only in spiritual drunkenness could he conquer his susceptibility to the nihilistic suggestions of complacent and unimaginative beings. PART III Julia and Laurence were to dine with Mr. and Mrs. Hurst. Of late Laurence had shown an unusual measure of social punctiliousness. Julia realized that his new determination to see and be with people was a part of his resistance to suffering. She thought bitterly that his regard for the opinions of others was greater than his regard for her. Julia put on a thin summer gown, very simply made, a light green sash, and a large black hat. Her misery had pride in itself, but when she looked in the glass she was pleased, and it was difficult to preserve the purity of her unhappiness. As she descended the stairs at Laurence's side she felt guiltily the trivial effect of her becoming dress. She wanted him to notice her. I'm afraid we are late. His fine eyes, with their sharp far-away expression, rested on her without seeming to take cognizance of her. I hope not. Mrs. Hurst is a hostess who demands punctuality. He spoke to her as to a child. There was something cruel in his kindness. For fear of exposing himself he refused her equality.
If he would only love her—that is to say, desire her—Julia knew that she would be willing to make herself even more abject than she had been, and that it would hurt her less than his considerate obliviousness. Laurence had ordered a taxi-cab. The driver waited at the curbstone in the twilight. He turned to open the door for the two as they came out. Julia was avidly, yet resentfully, aware of his surreptitious admiration. She told herself that her sex was so beggared that she accepted without pride its recognition by a strange menial. It was a beautiful cool evening. The glass in the taxi-cab was down. The cold stale smell of the city, blowing in their faces, was mingled with the perfume of the fading flowers in the park through which they passed. The trees rose strangely from the long dim drives. Here and there lights, surrounded by trembling auras, burst from the foliage. Far off were tall illuminated buildings, and, about them, in the deep sky, the reflection was like a glowing silence. The wall of buildings had the appearance of retreating continually while the cab approached, as if the huge blank bulks of hotels and apartment houses, withdrawing, held an escaping mystery. Laurence scarcely spoke. Julia's sick nerves responded, with a feeling of expectation, to the vagueness of her surroundings. Her heart, beating terrifically in her breast, seemed to exist apart from her, unaffected by her depression and fatigue. It was too alive. She cried inwardly for mercy from it. Mrs. Hurst's home was a narrow, semi-detached house with a brown-stone front and a bow window. From the upper floor it had a view of the park. When Julia and Laurence arrived, a limousine and Mr. Hurst's racer were already drawn up before the place. There were lights in one of the rooms at the right, and, between the heavy hangings that shrouded its windows, one had glimpses of figures. Laurence said sneeringly, Hurst has arrived, hasn't he! Affluent simplicity in a brown-stone front. You are honored that Mrs. Hurst is carrying you to glory with her.
Julia said, But they really are quite helpless with their money, Laurence. Mrs. Hurst has a genuine instinct for something better. How ceremonious is this occasion anyway? I don't know whether I am equal to the frame of mind that should accompany evening dress. There will only be one or two people. Mrs. Hurst knows how we dislike formal parties. Mr. Hurst, waving the servant back, opened the front door himself. He was a tall, narrow-shouldered man with a thin florid face. His pale humorous blue eyes had a furtive expression of defense. His mouth was thin and weak. His manner suggested a mixture of braggadocio and self-distrust. He dressed very expensively and correctly, but there was that in his air which somehow deprecated the success of his appearance. His sandy hair, growing thin on top, was brushed carefully away from his high hollow temples. The hand he held out, with its carefully manicured nails, was stubby-fingered and shapeless. Well, well, Farley! How goes it? I've been trying to get hold of you. Want to go for a little fishing trip? He was confused because he had not spoken to Julia first. How d'ye do, Mrs. Farley? Think you could spare him for a few days? Mr. Hurst's greeting of Laurence was a combination of bluff familiarity and resentful respect. When he looked at Julia his eyes held hers in bullying admiration. Julia had never been able to say just where his elusive intimacy verged on presumption. Feeling irritated and helpless and sweetly sorry for herself, she lowered her lids. My—dear! Mrs. Hurst kissed Julia. How sweet you look! How do you do, Mr. Farley? It was nice of you to let Julia persuade you to come to us. We really feel you are showing your confidence in us. Julia, dear girl, tells me you have as much of an aversion to parties as Charles and I have. This will be a homely evening. Mr. and Mrs. Wilson are here, and there is a young Hindoo who has been giving some charming talks at the Settlement House. He speaks very poor English but he's so interested in America. He's only become
acquainted with a few American women. I want him to meet Julia. I think he'll amuse her too. Mrs. Hurst's short little person was draped in a black lace robe embroidered with jet. She squinted when she smiled. Minute creases appeared about her bright eyes. Her expression was gentle and deceitful. Her arms, protruding from her sleeve draperies, were thin, and their movements weak. Her wedding ring and one large diamond-encircled turquoise hung loosely on the third finger of her left hand. Her hands were meager and showed that her bones were very small and delicate. About her hollow throat she wore a black velvet band, and her cheeks, no longer firm, were, nevertheless, childishly full above it. Though she said nothing that justified it, one felt in her a sort of affectionate malice toward those with whom she spoke. In her flattering acknowledgment of Julia's appearance there was something insidiously contemptuous. Come away with me, child, and we'll dispose of that hat. Williams! She turned to the Negro servant whom Mr. Hurst had intercepted at the door. She nodded toward Mr. Farley. The Negro went forward obsequiously. Yes, Williams, take Mr. Farley's hat, Mr. Hurst said. Then, in humorous confidence, sotto voce, How about a drink, Farley? My wife has that young Hindoo here. This is likely to be a dry intellectual evening. That may suit you, but I have to resort to first aid. Want to talk to you about that fishing trip. Come on to my den with me. Shortly after this, Julia, descending the stairs with her hostess, found Laurence and Mr. Hurst in the hall again. Laurence, his lips twisted disagreeably, was listening with polite but irritating quiescence to Mr. Hurst's incessant high-pitched talk. Mr. Hurst, who had been surreptitiously glancing toward the shadowy staircase that hung above his guest's head, was quick to observe the approach of the women. He had always found fault with what he considered to be Julia's coldness, but he admired her tall figure and her fine shoulders. Hello, hello! Here they are!
Charles! Mrs. Hurst was whimsically disapproving. Why haven't you taken Mr. Farley in to meet our guests? You are an erratic host. Mr. Hurst moved forward. That's all right! That's all right! Farley and I had some strategic confidences. You take him off and show him your Hindoo. I want Mrs. Farley to come out and see my rose garden, out in the court. I'm going to have a few minutes alone with her before you conduct her to the higher spheres and leave me struggling in my natural earthly environment. I won't be robbed of a little tête-à-tête with a pretty woman, just because there's an Oriental gentleman in the house who can tell her all about her astral body. Did you ever see your astral body, Mrs. Farley? Boo! Mrs. Hurst waved him off and pushed Julia toward him. Go on, if she has patience with you. But mind you only keep her there a moment. I've told Mr. Vakanda she was coming and I'm sure he's already uneasy. Rose garden, indeed! It's quite dark, Charles! Come, Mr. Farley. Put this scarf about you, dear. She took a scarf up and threw it around Julia's shoulders. Ta-ta! Mr. Hurst came confidently to Julia, and they walked out together across a glass-enclosed veranda that was brilliantly lit. Descending a few steps they were among the roses. Autumn roses, said Mr. Hurst. The bushes drooped in vague masses about them. Here and there a blossom made a pale spot among the obscure leaves. Where the glow from the veranda stretched along the paths, the grass showed like a blue mist over the earth, and clusters of foliage had a carven look. The dark wall of the next house, in which the lighted windows were like wounds, towered above them. Over it hung the black sky covered with an infinite flashing dust of stars. Julia's face was in shadow, but her hair glistened on the white nape of her neck where the black lace scarf had fallen away. Mr. Hurst had made a large sum of money from small beginnings. He would have enjoyed in peace the sense of power it gave him, and the indulgence in fine wines and foods and expensive surroundings for which he lived, but his wife prevented it. He had married her
when they were both young and impecunious. She had been a school teacher in a mid-western city. She had managed to convince him that in marrying him she conferred an honor upon him, and she succeeded now in making him feel out of place and absurd in the environment which his efforts had created, which she, however, turned to her own use. Instead of flaunting his success in boastful generosity, according to his inclination, he found himself compelled to deprecate it. He had a secret conviction that he was a man to be reckoned with, but openly, and especially before his wife's friends, he ridiculed himself, perpetrating laborious and repetitious jokes at his own expense, just as she ridiculed him when they were alone. Mrs. Hurst was chiefly interested in what she considered culture, and in welfare work, and among her acquaintances referred to her husband affectionately as if he were a child. She had no connection which would give her the entrée to socially exclusive circles, and she was wise enough not to attempt pretenses which it would have been impossible for her to sustain. Her husband's friends were mostly selfmade and newly rich. She was affable to them but maintained toward them a mild but superior reserve. She expressed tolerantly her contempt of social ostentation and suggested that among Mr. Hurst's play-fellows she was condescending from her more vital and intellectual pursuits. Men who drank and played golf or poker between the hours of business considered her brainy, but a damned nice woman. She was generous to impecunious celebrities of whom she had been told to expect success. On one occasion when she and Mr. Hurst were sailing for England she was photographed on shipboard in the company of a popular novelist. The picture of the novelist, showing Mrs. Hurst beside him in expensive furs, appeared in a woman's magazine. She had never seen the man since, but she always referred to him as a charming person. She was frequently called upon to conduct drives for charity funds. At masquerade balls organized for similar purposes her name appeared with others better known and she could honestly claim acquaintance with women whose frivolous occupations she professed to despise. She was an assiduous attendant at concerts
and the public lectures which were given from time to time by men of letters or exponents of the arts. References to sex annoyed her. The vagueness of her aspirations sometimes led her into fits of depression and discouragement, but she had a small crabbed pride that prevented her from allowing any one—least of all, perhaps, her husband—to see what she felt. She was conscientiously attentive to children, but actually bored by them. She seldom thought of her own childhood, and she sentimentalized her past only when she reflected on her early girlhood and the instinctive longing for withheld refinements which had led her away from a sordid uncultured home into the profession of a teacher. Often her husband irritated her almost uncontrollably, but she never admitted that the moods he aroused in her had any significance. She was ashamed of him and called the feeling by other names. Mr. Hurst's frustrated vanity consoled itself somewhat when he was alone before his mirror, for even his wife admitted that he was distinguished looking. He consumed bottle after bottle of a prescription which, so a specialist assured him, would make his hair come back. Always gay and affectionate and generally liked, he had a secret sensitiveness that he himself was but half aware of, and which no one who knew him suspected. He had never abandoned the romantic hope that some day he would meet a woman who would understand him. It was his unacknowledged desire to have his wife's opinion of him repudiated that made him perpetually unfaithful to her. Years ago he had been astonished to discover that even the women whom his wife introduced him to, who looked down on his absence of culture, and whose intellectual earnestness really seemed to him grotesque, were quite willing to take him seriously when he made love to them. He was bewildered but elated in perceiving the vulnerability of those he was invited to revere. Once he learned this it awakened something subtle and feminine in his nature and tempted him to unpremeditated cruelties. Though his sex entanglements were, as a rule, gross and banal enough, and quickly succeeded one another, he treasured at intervals a plaintive conviction that some day he would meet the woman who had, as he
expressed it, the guts to love him. Musing on this, he found in it the excuse for all the unpleasing episodes in which he took part. Outwardly cynical, he was sentimental to the point of bathos. He had one fear that obsessed him, the fear of growing old, so that the woman, when she met him, might not be able to recognize him. He had always been a little afraid of Julia and had a secret desire, on the rare occasions when they met, to hurt her in some way that might force her to concede their equality. He called himself a mixture of pig and child and when he met any of his wife's high-brow friends he envied them and wanted to trick them into exhibiting something of the pig also. Julia was young and pretty. He sighed and wished her more human. He had never found her so charming as she seemed to-night. Under the accustomed stimulus of alcohol he relaxed most easily into a mood of affectionate self-pity. Without being drunk in any perceptible way, he loved himself and he loved every one, and his conviction of human pathos was strong. Julia's tense yet curiously subdued manner showed him that she was no longer oblivious to him. He fancied that there was already between them that sudden rapport which came between him and women who were sexually sensible of his personality. You aren't angry with me for taking you away like this? Julia said, How could I be? I wish all social gatherings were in the open. It seems terrible to shut one's self indoors on these beautiful nights. Charles Hurst was impelled to talk about himself. He did not know how to begin, and coughed embarrassedly. He imagined that Julia was ready to hear, and already he was grateful for the regard he anticipated. Don't mind if I light a cigar? I should like it. Don't smoke cigarettes, do you? Some of the ladies who come here shedding sweetness and light are hard smokers. Julia shook her head negatively. I don't. But you surely can't object, as a principle, to women smoking?
No. I think my objections are chiefly—chiefly what my wife—what Catherine would call esthetic. I'm not strong on principles of any sort. Don't take myself seriously enough. Julia could make out his nonchalant angular pose as he stood looking down at her. As he held a match to his cigar the glow on his face showed his narrow regular features, his humorously ridiculing mouth, and his pale eyes caught in an unconscious expression of fright. Julia said, I'm afraid you take yourself very seriously indeed, or you wouldn't be so perpetually on the defensive. Poor Mr. Hurst! This evening she could not bear to be isolated by conventional reserves, even with him. It flattered her unhappiness to feel that he was a child. And this evening it seemed to her desperately necessary that she touch something living which would respond involuntarily to the contact. Mr. Hurst was disconcerted. He took the cigar out of his mouth and examined the glowing tip which dilated in the dark as he stared at it. Tears had all at once come to his eyes. He wondered if he were drunker than he had imagined. The moment he suspected any one of a serious interest in him it robbed him of his aplomb. Don't read me too well, Mrs. Farley. You know I'm not really much of a person. Coarse-fibered American type. No interests beyond business and all that. Good poker player. Hell of a good friend—when you let him. But commonplace. Damn commonplace. Nothing worth while at all from your point of view. They strolled along the path further into the shadows. Julia was astonished by the ill-concealed emotion in Mr. Hurst's humorous voice. His transparency momentarily assuaged the tortures of her self-distrust. How can you say that? My human predilections are not narrowed down to any particular type, I hope. Oh, well, I know—you and Catherine—miles over my head, all of it. Lectures on the Fourth Dimension. Some girl with adenoids here the other night been studying 'Einstein'. Damned if it had done her any
good. Yes, what that gal needed was somebody to hug her. Julia was conscious that he was turning toward her. Crass outlook, eh? He laughed apologetically. She probably did, Julia said. They laughed together. Mr. Hurst felt all at once unreasoningly depressed. He wanted to touch her as a child wants to touch the person who pleases it. But the sophisticated element in his nature intervened. He despised his own simplicity. Do you find yourself getting anywhere in the pursuit of the good, the true, and the beautiful? Honestly now, Mrs. Farley. I've had the whole program shoved at me—not that Catherine isn't the best of women, bless her little soul. You know the life we tired business men lead pretty much resembles that of the good old steady pack horse that does the work. We dream about green pastures and all that, but never get much closer to it. And when you get to the end of things you begin to wonder if your plodding did anybody any good—if anything ever did anybody any good. I've got no use for cynicism—consider it damn cheap. Wish some time I was a little bit more of a cynic. But I'm lost. Hopelessly lost. I take a highball every now and then because my—I think my mind hurts. He halted suddenly and they were looking into each other's vague faces. This talk getting too damn serious, eh? Something about you to-night that invites a fellow to make a fool of himself. I hope not, Julia said. I like you for talking frankly. Oh, I'm not too damn frank. We can't afford it in this world of hard knocks. Now to you, now, I'm not saying all that I'd like to, by a jugful. Then you don't make as much of a distinction between me and the crowd as I hoped. Charles had let his cigar go out. He kept turning it over and over in his stiff fingers that she could not see. He felt that only when he held a woman in his arms and she was robbed of her conventional defenses could he speak openly to her. With other attractive women he had come quickly to a point like this where he wanted to talk of
his inner life. He imagined it would give him relief if he could touch Julia's dress and put his head in her lap. The terrible fear of revealing himself before his wife and her friends had stimulated his imagination toward abandon. When he was a child his mother had not loved him. She was a defiant person. She was ashamed of him because he allowed himself to be victimized by all the things against which she had futilely rebelled. He had felt himself despised though he had never understood the reason. His mother found continual fault with him and never petted him. One day a girl cousin much older than he had discovered him in a corner crying and had comforted him, and had allowed him to put his head in her lap. As he had never gotten over considering himself from a child's standpoint, his adult visions always culminated in a similar moment of release. Whenever he became sentimental about a woman he imagined that he would some day put his head in her lap. He had been, in his own mind, so thoroughly convicted of weakness that the development of strength no longer appealed to him as a means of self-fulfilment. He abandoned himself to an incurable dependence for which he had not as yet found a permanent object. It eased him when he could evoke the maternal in a mistress. Aren't we all— somewhat on the defensive toward each other? he said after a minute. Julia was reminded again of what she thought to be her own tragedy. She felt reckless and wanted some one into whom to pour herself. She imagined herself lost in the dark garden, crushed between the walls and bright windows of the houses. In some indefinable way she identified herself with the million stars, flashing and remote in the black distance of the sky that showed narrowly above the roofs. Yes, she said. And so uselessly. People are so pathetic in their determination not to recognize what they are. If we ever had the courage to stop defending ourselves for a moment— But none of us have, I'm afraid. She carried the pity which she had for herself over to him. She had noticed how thin his face was, that the bold gaze with which he looked at her was only an expression of concealment, and that there were strained lines at the corners of his
good-tempered mouth. Yes, in the depths of his pale eyes with their conscious glint of humor there was undoubtedly something eager and almost blankly disconcerted. Charles could not answer her at once. He threw his cigar aside. His hand trembled a little. I wonder how drunk I am, he said to himself. He decided that he was helpless in the clutch of his own impulses. He thought, A damn fool now as always. Have I got this woman sized up wrong? She's a dear. Here goes. Poor little thing! Gosh, I know she can't be happy with that self-engrossed ass she's married to! In his more secret nature he was proud of his own temerity. Damn it all, Mrs. Farley—Julia— He hesitated. I've queered myself right off by calling you Julia, haven't I? His laugh was forced and unhappy. He glanced over his shoulder toward the house. Julia was alarmed by the unexpected immanence of something she was trying to ignore. She kept repeating to herself, He's a child! Her thoughts grew more disconnected each instant. She wanted to go away, yet she half knew that she was demanding of Charles the very thing that terrified her. Of course not. Mrs. Hurst calls me Julia, why shouldn't you? Her tone was intended to lift their talk to a plane of unsexed naturalness. Yes, by George, why shouldn't I! She calls you that a good deal as if she were your mother. He paused. Did you know I'd reached the ripe old age of forty-one? (He was really forty-two.) It doesn't shock me. Well, I wish it did. I don't like to be taken so damn much for granted. (He wanted to tell her that Catherine was three years older than he, but his sense of fair play withheld him.) An old man of my age has no right to go around looking for some one to understand him, has he? Why not? I'm afraid we do that to the end of time, Mr. Hurst. Say, now, honestly, Mrs. Farley—Julia—I can't lay myself wide open to anybody who insists on calling me Mr. Hurst. I feel as if I were a
hundred and seven. He tried to ingratiate himself with his boyishness. I haven't any objection to calling you Charles. (Julia thought uncomfortably of Mrs. Hurst and, remembering her, was embarrassed.) Don't feel hurt if I'm not able to do it at once. Certain habits of thought are very hard to get rid of. And I suppose you've been in the habit of considering me in the sexless antediluvian class! You've forgotten that Laurence—that my husband is as old as you are. When Julia mentioned her husband, Charles's impetuosity was dampened. It upset him and made him unhappy. However, he was determined to sustain his impulses. Yes, I had. Silence. Charles wanted to cry. You know I appreciate it awfully that you are willing to enter into the holy state of friendship with an obvious creature like myself. Catherine says you're a wonderful woman, and she's a damned good judge—of her own kind, that is. I'm afraid she's flattered me. I wish you weren't so humble about our friendship. I am as grateful as you are for anything genuine. Yes, I'm too confounded humble. I know I am. Always was. You know I'm not really lacking in self-respect, Miss Julia. Of course you aren't. You seem to me one of the most self- respecting people I know. Charles was silent a long time. He knew that he was being carried away on a familiar current. By God, she means it! he said to himself. He would refuse to regard anything but the present moment. How does it happen you and I never came together like this before? I'd got into the habit of thinking you were one of these icy Dianas that had an almighty contempt for any one as well rooted in Mother Earth as I am.
Julia laughed uncomfortably. That's a mixed metaphor. Then she said seriously, I want to understand things—not to try to escape. It seems to me we must all go back to Mother Earth if we try to do that. She added, I'm afraid we are making ourselves delinquent. We mustn't abandon Mrs. Hurst and her guests altogether. They turned toward the veranda. They were walking side by side and inadvertently Charles's hand brushed Julia's. He caught her fingers. She made a slight gesture of repulsion which he scarcely observed. Then her hand was relinquished to him. Confound these social amenities! I thought you were going to be my mother- confessor, Miss Julia. Until he touched her hand he had been conscious of their human separateness and his sensuous impulses had been in abeyance. With the feel of her flesh, she became simply the woman he wanted to kiss, the possessor of a beautiful throat, and of mysterious breasts that compelled him familiarly through the dim folds of her white dress. His acquisitive emotion was savage and childlike. Here was a strange thing which menaced and invited him. He wanted to know it, to tear it apart so that he need no longer be afraid of it. Already he annihilated it and loved it for being subject to him. He leaned toward her and when she lifted her face to him he kissed her. He felt the shudder of surprise that passed over her. Julia—don't hate me. Child, I'm going to fall in love with you! I know it! His voice was smothered in her hair. He kissed her eyes and her mouth again. Trembling, Julia was silent. He wondered recklessly if she despised him, but while he wondered he could not leave her. He felt embittered toward her because she awakened his dormant sensuality and he supposed that women like her were superior to the necessities that left him helpless. Please! Julia said. When his mouth was pressed against hers she was suffocated by the same thrill of astonishment and despair which she had experienced when she first allowed Dudley Allen to take her. When she was able to speak she said, Oh, we are so pathetic and absurd—both of us! It's so hopelessly meaningless.
He was excited and elated. In a broken voice, he said, So you think I am pathetic and absurd? I am, child. I don't care! I don't care! He thought that she was referring to the general opinion of him. He hardened toward her, while, at the same moment, a wave of physical tenderness enveloped him. Stealthily, he exulted in the capacity he possessed for sexual ruthlessness. He knew she could not suspect it. He would be honest with her only when it became impossible for her to evade him. They heard footsteps and turned from each other with a common instinct of defense. Mrs. Hurst was descending the steps from the lighted porch. I have a bone to pick with that spouse of mine, she called pleasantly when she could see them. Charles had taken out a fresh cigar and was lighting a match. Hello, hello! Am I in trouble again? Charles fumbled for Julia's hand, and gave it a squeeze, but dropped it as his wife drew near. Mrs. Hurst's figure was in silhouette before them. You'll spoil my dinner party, Charles! Julia, child, I'm afraid you need reprimanding too. You have to be stern with Charles. Her tone was truly vexed, but so frankly so that it was evident she suspected nothing amiss. I'm sorry if I am in disfavor. Julia's voice was cold. In her nihilistic frame of mind she wished that her hostess had discovered the compromising situation. Julia's reply was irritating and Mrs. Hurst's displeasure inwardly deepened. She felt stirring in her a chronic distrust and animosity toward other women, but would give no credence to her own emotion. Come, child, don't be ridiculous! I suppose I can't blame Charles for trying to steal you from me. I'm sure he wanted to talk to you about himself. It's the one thing he cannot resist. She laughed, a forced pleasant little laugh, and caught Julia's arm in a determined caressing pressure. Come. We're all going to be good. Mr. Vakanda is waiting to take you in to dinner. Julia followed her toward the house. Come, Charles! Mrs. Hurst commanded him
abruptly over her shoulder. The manner in which she spoke to him suggested strained tolerance. Charles's immediate relief at not having been seen was succeeded by complacency. To deceive his wife was for him to experience a naïve sense of triumph. Poor little Kate! He could even be sorry for her. Julia more than ever wanted to feel that Laurence's refusal of her was forcing upon her a promiscuous and degrading attitude toward sex. She said, I'm sure the fault is mine. I couldn't resist the night and the roses. Now don't try to defend him. The roses were his excuse, not yours. Mrs. Hurst wondered how they had been able to see anything of the roses in such a light. She wished to forget about it. Mollie Wilson has been telling us how difficult the role of a mother is these days. She says she envies you May with her amenability. Lucy has some of the most startlingly advanced conceptions of what her mother should let her do. Charles, walking almost on their heels, interrupted them. It would be an insult to Ju—to Mrs. Farley if I needed an excuse for carrying her off for a minute. He cleared his throat. Say, Kate, damn it all, will you and she be upset if I call her Julia? I like her as well as you do. Again Mrs. Hurst was irritated and inexplicably disturbed. It was Charles—not Julia—of course. Any woman. He's always like that! Then I shall expect to begin calling Mr. Farley Laurence, she said acidly. She spoke confidentially to Julia. He can't resist them, dear— any of them. Pretty women. You'll have to put up with his admiration. All my nicest friends do. The dickens they do! Charles grumbled jocosely. His wife's tone made him nervous. He was suspicious of her. When they came up on the lighted veranda a maid passed them, a neat good-looking young woman in black with inquisitive eyes. Julia caught on the servant's face what seemed an expression of inquiry
and amusement. Charles, who had often tried to flirt with the girl, glanced at her shamefacedly and immediately lowered his gaze. Damn these women! Julia, feeling guilty and antagonistic, observed Mrs. Hurst, but found that she appeared as usual, sweet and negatively self-contained, yet suggesting faintly a hidden malice. They walked through a long over-furnished hall and entered the drawing room. The men rose: the Hindoo, good-looking but with a softness that would inevitably repel the Anglo-Saxon; Mr. Wilson, stout and jovial, his small eyes twinkling between creases of flesh, the bosom of his shirt bulging over his low-cut vest; Laurence, clumsy in gesture, kind, but almost insulting in his composure. During the evening Julia could not bring herself to meet Laurence's regard, nor did she again look directly at Mr. Hurst. Charles, after some initial moments of readjustment when he found it difficult to join in the general talk, recovered himself with peculiar ease. Indeed his later manner showed such pronounced elation that Julia wondered if it were not eliciting some unspoken comment. When he turned toward her she was aware of the furtive daring of his expression, though she refused to make any acknowledgment of it. He laughed a great deal, made boisterous jokes uttered in the falsetto voice he affected when he was inclined to comicality, and, when his jests were turned upon himself, chuckled immoderately in appreciation of his own discomfiture. The Hindoo, whose bearing displayed extraordinary breeding, had opaque eyes full of distrust. His good nature under Charles's jibes was assumed with obvious effort and did not conceal his polite contempt. During dinner and afterward Charles plied every one, and particularly the men, with drink. Mrs. Hurst had always been divided between the attractions of the elegance which demanded a fine taste in wines and liqueurs, and her moral aversion to alcohol. She never served wines when she and Charles were alone, and to-night she was provoked by his ill- bred insistence that the glasses of her guests be refilled. When the meal was over and the men had returned to the drawing room, Charles seemed to be in a state of fidgets. His face and even
his helpless-looking hands were flushed. He walked about continually, and was perpetually smoothing his carefully combed hair over the baldish spot on the top of his head. Mrs. Wilson, who was florid and coarsely good-looking, with her iron-gray hair, admired his distinguished figure in its well-cut clothes. His flattering manner when he talked to her made her feel self-satisfied. Julia, though she had honestly protested to Charles that she did not smoke, indulged in a cigarette. Mrs. Wilson also lit one and expelled the smoke from her pursed mouth in jerky unaccustomed puffs. Mrs. Hurst's dislike of tobacco was equal to her repugnance to alcohol. She refused to smoke but was careful to show that her distaste for cigarettes was a personal idiosyncrasy. She made little amused grimaces at the smokers and treated them as if they were irresponsible children. Mrs. Wilson, in talking to Mr. Vakanda, contrived many casual and contemptuous references to her recent experiences in Europe. She was divided between her genuine boredom with European culture and her pride in her acquaintance with it. Charles, observing Julia in this group, appreciated the distinction of her simpler, more aristocratic manner; and the clarity and frankness of her statements seemed to him to place her as a being from another world. Damn me, she's a thoroughbred! Makes me ashamed of myself, bless her soul! His emotions were too much for him. He went into his den, which was across the hall, and poured himself a drink. Fragments of the evening's conversation buzzed in his head. Julia and Mr. Wilson had disagreed as to the validity of certain phases of the newer movements in art. Mr. Wilson scoffed blatantly at all of them. Mr. Vakanda was more reserved, but one suspected that he looked upon Westerners as adolescent and treated their art accordingly. Charles, without knowing what he was talking about, had come jestingly to Julia's rescue. When he remembered how often he had joined Mr. Wilson in ribald comment on subjects which she treated as serious, he felt he had been a traitor to her. Damn my soul, I'm hard hit! I never half appreciated that girl until to-night! Don't know what the hell's been the matter with me! Overcome by his reflections, he walked to a window and stared out into the quiet
dimly lit street. His suddenly aroused sensual longing for Julia returned and made him embarrassed and unhappy. He set his glass down on the window ledge and passed a hand across each eye as if he were wiping something away. Damn it all, I'm in love with her all right. When the time for the Farleys' departure arrived Charles was talkative and uneasy. He clapped his hand on Laurence's shoulder. You're one of the few men who's fit to fish with, Farley. Most of 'em are too damned loud for the fish. We'll fix that little trip up yet. I suspect you of being the philosopher of this bunch anyway. I can furnish the requisite of silence, but I'm afraid it requires some peculiar psychic influence to attract fish. I haven't got it. Charles's manner was self-conscious to a degree. He spoke rapidly and unnecessarily lifted his voice. His wife watched him with a cold kind little smile of disgust. She wanted to create the impression that she understood him, but her resentment of him rose chiefly from the fact that he was incomprehensible to her. That's all right. I'll catch the fish. I'll catch the fish. Damned if I haven't enjoyed the evening. Say, Farley, Kate and I are coming over some evening and I'm going to talk to your wife. I believe she's just plain folks even if she can chant Schopenhauer and the rest of those cranks. You know I admire your brains, Miss Julia. By Jove, I do. You can give me some of the line of patter I've missed. Kate, now—Kate's got it all at her finger tips, but she's given me up long ago. Have a drink before you go, Farley? No! You know I'm a great admirer of Omar Khayyám's, Miss Julia. The rest of you high-brows seem to have put the kibosh on the old boy. He's the fellow that had some bowels of compassion in him. Knew what it was like to want a drink and be dry. Charles smoothed back his hair. His hand was trembling slightly. He looked at Julia now and then but allowed no one else to catch his eyes.
Laurence, holding his silk hat stiffly in his fingers, moved determinedly toward the front door. His smile was enigmatic but his desire for escape was evident. Julia said, I'll talk to you about Schopenhauer, Mr. Hurst, and convince you that he was very far from a crank. She smiled. Yep? Well, guess I'm jealous of him. I'm willing to be taught. This business grind I'm in is converting me into pretty poor company. Not much use for a meditative mind in the stock market. Eh, Farley? The women have got it all over us when it comes to refining life. Laurence said, I imagine I know as little of the stock market as my wife, Hurst. And you must remember I'm a business woman, too. So you are. Working in that confounded laboratory. Well, I've got no excuse then. Know thyself, Charles! Mrs. Hurst shook her finger playfully. Yep. Constitutional aversion to knowing myself—knowing anything else. Looks to me as if you had picked a lemon, Kate. We must really go. Julia held out her hand. Mrs. Hurst shook hands with Julia. So delightful to have had you. I'm glad you impressed Mr. Vakanda with the significance of America in the world of art, dear. Mrs. Hurst, at that instant, disliked her guest intensely, but she preserved her smile and her delicate tactful air. Laurence shook hands with her also. His reserve appealed to her. She could be more frankly gracious with him. Charles pressed Julia's fingers lingeringly, in spite of her efforts to withdraw them. He was suddenly depressed and gazed at her with an open almost despairingly interrogative expression. Yep, damn me, Kate's right. You put the Far East in its place, Miss Julia. Did me good to see it. He giggled nervously, but his face immediately grew serious. Seeing her go away into her own strange world depleted the confidence he experienced while with her. He was oppressed by the
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The Physics And Chemistry Of The Interstellar Medium A G G M Tielens

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    The Physics andChemistry of the Interstellar Medium This work provides a comprehensive overview of our current theoretical and observational understanding of the interstellar medium of galaxies. With emphasis on the microscopic physical and chemical processes in space, and their influence on the macroscopic structure of the interstellar medium of galaxies, the book includes the latest developments in this area of molecular astrophysics. The various heating, cooling, and chemical processes relevant for the rarefied gas and submicron-sized dust grains that constitute the interstellar medium are discussed in detail. This provides a firm foundation for an in-depth understanding of the ionized, neutral atomic, and molecular phases of the interstellar medium. The physical and chemical properties of large polycyclic aromatic hydrocarbon molecules and their role in the interstellar medium are highlighted, and the physics and chemistry of warm and dense photodissociation regions are discussed. This is an invaluable reference source for advanced undergraduate and graduate students and research scientists. Alexander Tielens is a professor of astrophysics at the Kapteyn Astronomical Institute in the Netherlands, and a senior scientist with the Dutch space agency, SRON. Prior to this, he has worked as an assistant researcher in the Astronomy Department of the University of California, Berkeley, and as a senior scientist at NASA Ames Research Center, California. He has published extensively on various aspects of the physics and chemistry of the interstellar medium.
  • 9.
    THE PHYSICS ANDCHEMISTRY OF THE INTERSTELLAR MEDIUM A.G.G.M. TIELENS Kapteyn Astronomical Institute
  • 10.
       Cambridge,New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge  , UK First published in print format - ---- - ---- © A. G. G. M. Tielens 2005 2005 Information on this title: www.cambridge.org/9780521826341 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. - --- - --- Cambridge University Press has no responsibility for the persistence or accuracy of s for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Published in the United States of America by Cambridge University Press, New York www.cambridge.org hardback eBook (NetLibrary) eBook (NetLibrary) hardback
  • 11.
    Contents Preface page ix Listof constants xi List of conversion factors xii 1 The galactic ecosystem 1 1.1 Interstellar objects 2 1.2 Components of the interstellar medium 6 1.3 Energy sources 12 1.4 The Milky Way 18 1.5 The mass budget of the ISM 20 1.6 The lifecycle of the Galaxy 22 1.7 Physics and chemistry of the ISM 22 1.8 Further reading 23 2 Gas cooling 25 2.1 Spectroscopy 25 2.2 Cooling rate 45 2.3 Two-level system 46 2.4 Gas cooling in ionized regions 53 2.5 Gas cooling in neutral atomic regions 54 2.6 Cooling law 58 2.7 Further reading 60 3 Gas heating 63 3.1 Overview 63 3.2 Photo-ionization of atoms 64 3.3 Photo-electric heating 66 3.4 Photon heating by H2 71 3.5 Dust-gas heating 74 3.6 Cosmic-ray heating 75 v
  • 12.
    vi Contents 3.7 X-rayheating 76 3.8 Turbulent heating 77 3.9 Heating due to ambipolar diffusion 79 3.10 Gravitational heating 80 3.11 The heating of the interstellar medium 80 3.12 Further reading 83 4 Chemical processes 85 4.1 Gas-phase chemical reactions 85 4.2 Grain-surface chemistry 101 4.3 Further reading 114 5 Interstellar dust 117 5.1 Introduction 117 5.2 Physical processes 118 5.3 Observations 145 5.4 The sizes of interstellar grains 157 5.5 The composition of interstellar dust 161 5.6 Further reading 169 6 Interstellar polycyclic aromatic hydrocarbon molecules 173 6.1 Introduction 173 6.2 IR emission by PAH molecules 175 6.3 PAH charge 190 6.4 Photochemistry of PAHs 198 6.5 Other large molecules 209 6.6 Infrared observations 212 6.7 The IR characteristics of PAHs 213 6.8 Further reading 224 7 HII regions 228 7.1 Overview 228 7.2 Ionization balance 228 7.3 Energy balance 246 7.4 Emission characteristics 251 7.5 Comparison with observations 256 7.6 Further reading 263 8 The phases of the ISM 265 8.1 Introduction 265 8.2 Physical processes in atomic gas 266 8.3 The CNM and WNM phases of the ISM 271 8.4 The warm ionized medium 276
  • 13.
    Contents vii 8.5 Thehot intercloud medium 277 8.6 Summary: the violent ISM 285 8.7 Chemistry of diffuse clouds 287 8.8 The cosmic-ray ionization rate 299 8.9 Observations 304 8.10 Further reading 314 9 Photodissociation regions 317 9.1 Introduction 317 9.2 Ionization balance 319 9.3 Energy balance 320 9.4 Dust temperature 324 9.5 Chemistry 326 9.6 PDR structure 329 9.7 Comparison with observations 331 9.8 PDR diagnostic model diagrams 336 9.9 The Orion Bar 339 9.10 Physical conditions in PDRs 343 9.11 Hydrogen IR fluorescence spectrum 344 9.12 Further reading 345 10 Molecular clouds 348 10.1 Introduction 348 10.2 The degree of ionization 349 10.3 Energy balance 351 10.4 Gas-phase chemistry 353 10.5 Grain-surface chemistry 364 10.6 Gas–grain interaction 369 10.7 Observations 377 10.8 Further reading 392 11 Interstellar shocks 396 11.1 Introduction 396 11.2 J-shocks 397 11.3 C-shocks 408 11.4 Further reading 416 12 Dynamics of the interstellar medium 417 12.1 Introduction 417 12.2 The expansion of HII regions 418 12.3 Supernova explosions 440 12.4 Supernovae and the interstellar medium 447 12.5 Interstellar winds 453
  • 14.
    viii Contents 12.6 Thekinetic energy budget of the ISM 458 12.7 Further reading 459 13 The lifecycle of interstellar dust 461 13.1 Introduction 461 13.2 Shock destruction 463 13.3 Dust lifetimes 468 13.4 The grain size distribution 470 13.5 Dust abundances and depletions 470 13.6 Mass balance of interstellar dust 474 13.7 Further reading 474 14 List of symbols 476 Index of compounds 484 Index of polycyclic aromatic hydrocarbons 485 Index of molecules 487 Index of objects 490 Index 491
  • 15.
    Preface When, upon myreturn to Holland, I started to teach an advanced course on the interstellar medium in 1998, I quickly realized that there was no suitable textbook available. There is, of course, the incomparable monograph by Spitzer, Physics of the Interstellar Medium (1978, New York: Wiley and Sons). But that book is quite challenging and not very suitable for a student course. Moreover, by now, it is very dated. Over the intervening years, our insights into the basic physics of the interstellar medium have much improved thanks, for example, to the opening up of the infrared and submillimeter windows. In particular, molecules, which we now know to be deeply interwoven into the fabric of the Universe, play only a little role in Spitzer’s book. When Eddington made his famous remark, “Atoms are physics but molecules are chemistry,” he merely expressed, on the one hand, the dream of a physicist of a simple universe, which can be caught in a single equation, and, on the other hand, the dread of a reality where solutions are never clean and simple. The latter is of course obvious to a chemist and it is now abundantly clear that Eddington’s fear has turned into reality, even for astronomy. Present-day graduate students will require an intimate knowledge of molecular astrophysics in order to be active in the field of the interstellar medium of our own or other galaxies whether it is in the here and now or all the way back in the early Universe. This will become even more the case with the launch of the submillimeter space mission, Herschel, in 2007, when the Atacama Large Millimeter Array is finished in 2011, and with the launch of the James Webb Space Telescope in the next decade. Together these missions will push the frontier of the molecular Universe all the way back to the initial pollution of the Universe with the first metals by the first generation of luminous objects, which forever spoiled the physicist’s Garden of Eden. This book covers both the physics and the chemistry of the interstellar medium. Chapters on heating, cooling, and chemical processes provide the students with the necessary toolbox for the astrophysics and astrochemistry of the interstel- lar medium. This background is rounded off with chapters on the physics and ix
  • 16.
    x Preface chemistry ofinterstellar dust and large molecules. Once the students have mastered these subjects, they are well prepared for an in-depth discussion of classical topics of the interstellar medium: HII regions, the phases of the interstellar medium, shocks, and the dynamical interaction of HII regions, supernova remnants, and stellar winds with the ISM. The chemistry of the interstellar medium is covered in chapters on diffuse clouds, photodissociation regions, and molecular clouds. All together, this forms a comprehensive course, covering most current aspects of the interstellar medium, which will prepare students well for the future. Over the years, this book grew from the course that I taught in Groningen. Indeed, in many ways, writing this book carried me through those dark Dutch days. Fortunately, I have many good friends who understand that, when the Sun sets in October not to appear again until May, it is a good time to leave Holland and visit other institutes. I owe a deep debt of gratitude to the Miller Institute and the Astronomy Department of the University of California in Berkeley and my hosts Imke de Pater and Chris McKee, to the Space Sciences Division of NASA Ames Research Center and my hosts David Hollenbach and Lou Allamandola, to the Institute for Geophysics and Planetary Physics of the Lawrence Livermore National Laboratory and my hosts Wil van Breugel and John Bradley, to the Centre d’Etudes Spatiale des Rayonnements in Toulouse and my host Emmanuel Caux, and to the Laboratoire d’Astrophysique de l’Observatoire de Grenoble and my host Cecilia Ceccarelli for their hospitality and for providing an environment conducive to great science. Most of the chapters of this book were conceived and written during these extended visits. Of course, much of this book reflects a lifetime spent in discovering the molecular Universe. I want to thank Harm Habing without whose human touch I would have left astronomy before even finishing the first stage of my journey. Also, I owe much to Lou Allamandola and David Hollenbach, with whom I have spent so many wonderful hours on the trail of discovery: not only for sharing their deep insights and understanding of physical and chemical processes of relevance to studies of the interstellar medium but, particularly, for their friendship. I am also deeply indebted to the many graduate students, who carried me through the many stages of this course, solved the many L ATEX problems, and always succeeded in making the right figures, as well as for their careful proofreading of the manuscript. Most of all, their enthusiasm always managed to perk me up. In this regard, I specifically want to thank Adwin Boogert, Rense Boomsma, Jan Cami, Stephanie Cazaux, Sasha Hony, Jacquie Keane, Leticia Martín-Hernández, Chris Ormel, Els Peeters, and Henrik Spoon for their help. Finally, Marion understands like no other what it is to live away from what feels like home. Her encouragement to follow my dream and her support during these difficult years have made this possible. To my girls–Anneke, Saskia, and Elske–the only thing I can say is that, now, it is really done.
  • 17.
    Constants Physical constants Symbol DescriptionSI cgs Value Unit Value Unit c Speed of light 29979 8 m s−1 29979 10 cm−1 s−1 h Planck’s constant 66261−34 J s 66261−27 erg s k Boltzmann’s constant 13807−23 J/K 13807−16 erg/K SB Stefan–Boltzmann constant 56704 −8 W m−2 K−4 56704 −5 erg s−1 cm−2 K−4 G Gravitational constant 6674 −11 N m−2 kg−2 6674 −8 dyn cm−2 g−2 NA Avogadro’s constant 60221 23 mol−1 60221 23 mol−1 me Electron rest mass 91094−31 kg 91094−28 g mp Proton rest mass 16726−27 kg 16726−24 g mu Atomic mass unit 16605−27 kg 16605−24 g e Electron charge 1602 −19 C 4803 −10 esu Fine-structure constant 72974 −3 72974 −3 Values a×10b are given as a (b). Astronomical constants Symbol Description SI cgs Value Unit Value Unit AU Astronomical unit 1496 11 m 1496 13 cm ly Light year 9463 15 m 9463 17 cm pc Parsec 3086 16 m 3086 18 cm pc2 Square parsec 95234 32 m2 95234 36 cm2 kpc2 Square kiloparsec 95234 38 m2 95234 42 cm2 L Solar luminosity 385 26 J s−1 385 33 erg s−1 M Solar mass 1989 30 kg 1989 33 g R Solar radius 696 8 m 696 10 cm T Solar effective temperature 578 3 K 578 3 K Jy Jansky 100 −26 W m−2 H z−1 100 −23 erg s−1 cm−2 Hz−1 Values a×10b are given as a (b). xi
  • 18.
    Conversion factors Angles andlengths Unit/symbol Description SI cgs Value Unit Value Unit deg degree 17453 −2 rad 17453 −2 rad arcmin arcminute 290888−4 rad 290888−4 rad arcsec arcsecond 48481 −6 rad 48481 −6 rad sq deg degree2 3046 −4 sr 3046 −4 sr Å angstrom 10 −10 m 10 −8 cm m micrometer 10 −6 m 10 −4 cm Values a×10b are given as a (b). SI and cgs units Description SI cgs Value Unit Value Unit Time 1 s 1 s 1 year 3.16 (7) s Length 1 m 1 (2) cm Velocity 1 m s−1 1 (2) cm s−1 Force 1 N 1 (5) dyne Pressure 1 Pa 1 (−1) dyne cm−2 Energy 1 J 1 (7) erg Charge 1 C 2.9979 (9) esu Magnetic flux density 1 T 1 (4) gauss Values a×10b are given as a (b). xii
  • 19.
    List of conversionfactors xiii Energy conversion factors erg eV K cm−1 Hz erg 100 6242 11 7243 15 5034 15 1509 26 eV 1602 −12 100 11604 4 80644 2418 14 K 13806−16 8617 −5 100 0695 2084 10 cm−1 19865−16 1240 −4 14389 100 2997010 Hz 6626 −27 4136−15 4798 −11 3336−11 100 Values a × 10b are given as a (b). To convert from unit in column 1 to units above the rows, multiply by value; e.g., 1eV = 1602×10−12 erg. A useful compendium of constants can be found in C. W. Allen, Astrophysical Quantities, (London: The Athlone Press). The website http://physics.nist.gov/cuu/, maintained by the National Institute of Standards and Technology, provides a wealth of information on constants.
  • 21.
    1 The galactic ecosystem TheMilky Way is largely empty. Stars are separated by some 2 pc in the solar neighborhood = 6 × 10−2 pc−3. If we take our Solar System as a measure, with a heliosphere radius of 235 AU, stars and their associated planetary systems fill about 3×10−10 of the available space. This book deals with what is in between these stars: the interstellar medium (ISM). The ISM is filled with a tenuous hydrogen and helium gas and a sprinkling of heavier atoms. These elements can be neutral, ionized, or in molecular form and in the gas phase or in the solid state. This gas and dust is visibly present in a variety of distinct objects: HII regions, reflection nebulae, dark clouds, and supernova remnants. In a more general sense, the gas is organized in phases – cold molecular clouds, cool HI clouds, warm intercloud gas, and hot coronal gas – of which those objects are highly visible manifestations. This gas and dust is heated by stellar photons, originating from many stars (the so-called average interstellar radiation field), cosmic rays (energetic [∼GeV] protons), and X-rays (emitted by local, galactic, and extragalactic hot gas). This gas and dust cools through a variety of line and continuum processes and the spectrum will depend on the local physical conditions. Surveys in different wavelength regions therefore probe different components of the ISM. This first chapter presents an inventory of the ISM with an emphasis on prominent objects in the ISM and the global structure of the ISM. The interstellar medium plays a central role in the evolution of the Galaxy. It is the repository of the ashes of previous generations of stars enriched by the nucleosynthetic products of the fiery cauldrons in the stellar interiors. These are injected either with a bang, in a supernova explosion, or with a whimper, in the much slower moving winds of low-mass stars on the asymptotic giant branch. In this way, the abundances of heavy elements in the ISM slowly increase. This is part of the cycle of life for the stars of the Galaxy, because the ISM itself is the birthplace of future generations of stars. It is this constant recycling and its 1
  • 22.
    2 The galacticecosystem Figure 1.1 A panoramic image of a (southern) portion of the Milky Way’s disk. The image has been inverted and dark corresponds to emission from ionized gas and reflection nebulae. The light band stretching irregularly across the whole image is due to absorption by dust clouds. Image courtesy of J. P. Gleason. associated enrichment that drives the evolution of the Galaxy, both physically and in its emission characteristics. 1.1 Interstellar objects 1.1.1 HII regions Ionized gas nebulae feature prominently in the Milky Way as bright visible nebulous objects. The Great Nebula in Orion (M42; Fig. 1.2) and the Lagoon Nebula (M8) are well-known examples. HII regions span a range in brightness, however, and fainter examples are the California Nebula and IC 434 (Fig. 1.3). The gas in these regions is ionized and has a temperature of about 104 K. Densities range from 103 –104 cm−3 for compact (∼05 pc) HII regions such as the Orion Nebula to ∼10 cm−3 for more diffuse and extended nebulae such as the North America Nebula (∼10 pc). The optical spectra of these regions are dominated by H and He recombination lines and collisionally excited, optical (forbidden) line emission from trace ions such as [OII], [OIII], and [NII]. HII regions are also strong sources of thermal radio emission (free–free) from the ionized gas and of infrared emission due to warm dust. HII regions are formed by young massive stars with spectral type earlier than about B1 (Teff 25000 K), which emit copious amounts of photons beyond the Lyman limit (h 136 eV) and ionize and heat their surrounding, nascent molecular clouds. They are, therefore, signposts of sites of massive star formation in the Galaxy. 1.1.2 Reflection nebulae Reflection nebulae are bluish nebulae that reflect the light of a nearby bright star. NGC 2023 in the Orion constellation (see Fig. 1.3) and the striated nebulosity associated with the Pleiades are familiar cases. In this case, the observed light
  • 23.
    1.1 Interstellar objects3 Figure 1.2 A black and white representation of the Orion Nebula as observed by the Hubble Space Telescope in [OIII], H, and [NII]. Light and dark have been inverted. The gas is ionized by the Trapezium cluster, in particular 1 C Ori, in the center of the image. The bright bar in the south-west is an ionization front eating its way into the surrounding neutral material in the photodissoci- ation region known as the Orion Bar. The dark bay (light in this image), a cloud of foreground obscuring material, is also evident to the east. This image gives a clear view of the complex topography created by the interaction of a newly formed massive star with its surrounding natal cloud. Image courtesy of R. O’Dell. is not due to hot gas but rather reflected starlight. There is no radio emission but there is infrared emission from warm dust, although this is less luminous than for HII regions. For the compact reflection nebulae, densities are typically a little smaller (103 cm−3 ) than for compact HII regions. Reflection nebulae are illuminated by stars with spectral types later than about B1. Regions around hotter stars also show (faint) reflected light emission but the spectrum is then dominated by the emission from the ionized gas. For the earlier stellar types, the surrounding nebulosity may be the material from which the star was formed (e.g., NGC 2023; NGC 7023). Often, however, the nebulosity is due to a chance encounter between the star and a cloud (e.g., the Pleiades). Reflection nebulae can also be associated with the ejecta of a late-type star (e.g., IC 2220; the Red Rectangle).
  • 24.
    4 The galacticecosystem Figure 1.3 Part of the Orion molecular cloud containing the Horse Head Nebula. The diffuse glow behind the horse head is IC 434 ionized by the bright star, Ori. The horse head is a protrusion of the molecular cloud obvious in the lower part of this image. The nebula to the south-east of the horse head is the reflection nebula, NGC 2023. Image courtesy of the Canadian-France-Hawaii telescope, J.-C. Cuillandre, Coelem. 1.1.3 Dark nebulae A striking aspect of the all-sky optical view of the Milky Way is the presence of many dark regions in which few stars are seen (cf. Fig. 1.1). The direction towards the center of the Galaxy is rampant with such dark clouds, which actually seem to divide the galactic plane in two. The Coalsack near the Southern Cross is a particularly nice example of a roundish dark cloud. Dark clouds are readily apparent when backlighted. The Horse Head Nebula (see Fig. 1.3) silhouetted against the reddish glow of the HII region, IC 434, and the dark bay in the Orion HII region (see Fig. 1.2) are two famous examples. Individual dark clouds come in a range of sizes from tens-of-parsecs large to the tiny (∼10−2 pc) Bok globules associated with HII regions such as the Orion Nebula. Likewise, some dark clouds are completely black (Av 10 magnitudes) while others are hardly discernible. While dark clouds are outlined by the absence of stars, they do show faint optical
  • 25.
    1.1 Interstellar objects5 reflected light. Also, they become bright at mid- and far-infrared wavelengths. Some really dense clouds are opaque even at mid-IR wavelengths and appear as infrared dark clouds (IRDC) in absorption against background galactic mid-IR emission. 1.1.4 Photodissociation regions While HII regions and reflection nebulae dominate the Galaxy at visible wave- lengths, in the infrared, photodissociation regions dominate the sky. Originally, the name photodissociation regions (PDRs; sometimes also called photodominated regions with fortunately the same abbreviation) was given to the atomic–molecular zones that separate ionized and molecular gas near bright luminous O and B stars (e.g., surrounding HII regions and reflection nebulae) and the Orion Bar (Fig. 1.2) and NGC 2023 (Fig. 1.3) are prime examples of classical PDRs. In these regions, penetrating far-ultraviolet (FUV) photons (with energies between 6 and 13.6 eV) dissociate and ionize molecular species. Most of the FUV photons are absorbed by the dust, but a small fraction heat the gas through the photo- electric effect to a few hundred degrees. Photodissociation regions are thus bright in IR dust continuum and atomic fine-structure cooling lines as well as molecular lines. In essence, of course, everywhere where FUV photons strike a cloud, a PDR will ensue. Indeed, the term PDRs has now expanded to include all regions of the ISM where FUV photons dominate the physical and chem- ical processes. As such, PDRs include the neutral atomic gas of the ISM as well as much of the gas in molecular clouds (except, e.g., for dense starless cores). 1.1.5 Supernova remnants Supernova remnants (SNRs) are formed when the material ejected in the explosion that terminates the life of some stars shocks surrounding ISM material and an SNR’s spectrum is that of a high velocity shock. About 100 supernova remnants are visible in our Galaxy; they are generally characterized by long, delicate fila- ments radiating in-line radiation (Fig. 1.4). Supernova remnants are prominent sources of radio emission due to relativistic electrons spiraling around a magnetic field (synchroton emission) and some 200 have been identified at radio wave- lengths. Supernova remnants also stick out at X-ray wavelengths because of emission by hot (106 K) gas. Not all SNRs are wispy. The Crab Nebula is an example of a compact SNR.
  • 26.
    6 The galacticecosystem Figure 1.4 A small portion of the Cygnus Loop, the remnant of a supernova that exploded about 10 000 years ago. The image has been inverted to bring out the delicate structure of the nebulosity. The emission is due to a shock wave and is about 3 pc in size. Image courtesy of L. K. Tan, StarryScapes. 1.2 Components of the interstellar medium The gas in the ISM is organized in a variety of phases. The physical properties of these phases are summarized in Table 1.1. 1.2.1 Neutral atomic gas The 21 cm line of atomic hydrogen traces the neutral gas of the ISM. This neutral gas can also be observed in optical and UV absorption lines of various elements towards bright background stars. The neutral medium is organized in cold ( 100 K) diffuse HI clouds (cold neutral medium, CNM) and warm (≈8000 K) intercloud gas (warm neutral medium, WNM). A standard HI cloud (often called a Spitzer-type cloud) has a typical density of 50 cm−3 and a size of 10 pc. The density of the WNM is much less (05 cm−3). Between 4 and 8 kpc from the galactic center, 80% of the HI mass in the plane of the Galaxy is in diffuse clouds in a layer with a (Gaussian) scale height of about 100 pc. At higher latitudes, however, much of the HI mass is in the intercloud medium with a larger scale height of 220 pc but with an exponential tail extending well into the lower halo. These two neutral phases have, on average, similar surface densities. Because the Sun is located in the local bubble, the local, total WNM column density towards the North Galactic Pole is about 2.5 times that of the CNM. In the outer Galaxy, the HI scale height rapidly increases.
  • 27.
    1.2 Components ofthe interstellar medium 7 Table 1.1 Characteristics of the phases of the interstellar medium Phase na 0 (cm−3 ) Tb (K) c v (%) Md (109 M) n0 e (cm−3 ) Hf (pc) g Mpc−2 Hot intercloud 0.003 106 ∼50.0 — 00015 3000 03 Warm neutral medium 0.5 8000 30.0 28 01h 220h 15 006h 400h 14 Warm ionized medium 0.1 8000 25.0 10 0025i 900i 11 Cold neutral mediumj 50.0 80 1.0 22 04 94 23 Molecular clouds 200.0 10 0.05 13 012 75 10 HII regions 1–105 104 — 005 0015k 70k 005 a Typical gas density for each phase. b Typical gas temperature for each phase. c Volume filling factor (very uncertain and controversial!) of each phase. d Total mass. e Average mid-plane density. f Gaussian scale height, ∼ exp −z/H2 /2, unless otherwise indicated. g Surface density in the solar neighborhood. h Best represented by a Gaussian and an exponential. i WIM represented by an exponential. j Diffuse clouds. k HII regions represented by an exponential. 1.2.2 Ionized gas Diffuse ionized gas in the ISM can be traced through dispersion of pulsar signals, through optical and UV ionic absorption lines against background sources, and through emission in the H recombination line (see Fig. 1.5). The first two can only be done in a limited number of selected sight-lines. The faintness and large extent of the galactic H hamper the last probe. While most of the H luminosity of the Milky Way is emitted by distinct HII regions, almost all of the mass of ionized gas (109 M) resides in a diffuse component. This warm ionized medium (WIM) has a low density (01 cm−3 ), a temperature of ≈8000 K, a volume filling factor of 025, and a scale height of 1 kpc. The weakness of the [OI] 6300 line (in a few selected directions) implies that the gas is nearly fully ionized. The source of ionization is not entirely clear. Energetically, ionizing photons from O stars are the most likely candidates but these photons have to “escape” from the associated HII regions and travel over large distances (hundreds of parsecs)
  • 28.
    8 The galacticecosystem Wisconsin H-Alpha Mapper Northern Sky Survey Total Integrated Intensity Map (–80 VLSR +80 km s–1 ) l=120° ∆l=30° Log Intensity [Rayleighs] http://www.astro.wisc.edu/wham/ 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 –0.25 –0.50 ∆b=15° Figure 1.5 The integrated galactic ( versus b) H emission obtained by the WHAM survey. This emission from ionized gas, about a million times fainter than the Orion Nebula, traces the warm ionized medium. Note the large filament (40 ) sticking up out of the galactic plane. Image courtesy of R. Reynolds. without being absorbed by omnipresent neutral hydrogen. Finally, the WIM shows a complex spatial structure including thin filaments sticking ∼1 kpc out of the plane of the Galaxy, further compounding the ionization problem. 1.2.3 Molecular gas The CO J = 1−0 transition at 2.6 mm is commonly used as a tracer of molecular gas in the Galaxy (Fig. 1.6). Surveys in this line have shown that much of the molecular gas in the Milky Way is localized in discrete giant molecular clouds with typical sizes of 40 pc, masses of 4×105 M, densities of 200 cm−3 , and temperatures of 10 K. However, it should be understood that molecular clouds show a large range in each of these properties. Molecular clouds are characterized by high turbulent pressures as indicated by the large linewidths of emission lines. Molecular clouds are self-gravitating rather than in pressure equilibrium with other phases in the ISM. While they are stable over time scales of 3×107 years, presumably because of a balance of magnetic and turbulent pressure and gravity, molecular clouds are the sites of active star formation. Observations of molecular
  • 29.
    1.2 Components ofthe interstellar medium 9 Figure 1.6 The emission of CO in the outer Galaxy obtained by the FCRAO survey. Much of the emission is local, stemming from molecular clouds between 0.5 and 1 kpc. In addition, giant molecular clouds associated with the NGC 7538-Sharpless 156-Sharpless 152 region ( = 110 ) and the W3-W4-W5 region ( = 135 ) are also present. The image has been inverted and dark corresponds to emission. Image courtesy of Chris Brunt (FCRAO). clouds in the rotational transitions of a variety of species allow a detailed study of their physical and chemical properties. These studies show that molecular clouds have spatial structure on all scales. In particular, molecular clouds contain cores with sizes of 1 pc, densities in excess of 104 cm−3 , and masses in the range 10–103 M in which star formation is localized. While CO is commonly used to trace interstellar molecular gas, H2 is thought to be the dominant molecular species, with a H2/CO ratio of 104 –105 . In addition, some 200 different molecular species have been detected – mainly through their rotational transitions in the submillimeter wavelength regions – in the shielded environments of molecular clouds. In general, these species are relatively simple, often unsaturated, radi- cals, or ions. Acetylenic carbon chains and their derivatives figure prominently on the list of detected molecules. However, this may merely reflect observa- tional bias, since such species possess large dipole moments and relatively small partition functions, both of which make them readily detectable at microwave wavelengths. 1.2.4 Coronal gas Hot (∼105 –106 K) gas can be traced through UV absorption lines of highly ionized species (e.g., CIV, SVI, NV, OVI) seen against bright background sources. Such hot plasmas also emit continuum (bremsstrahlung, radiative recombination, two photon) and line (collisionally excited and recombination) radiation in the extreme ultraviolet and X-ray wavelength regions. These observations have revealed the existence of a pervasive hot (3–10 × 105 K) and tenuous (10−3 cm−3) phase (the hot intercloud medium, HIM) of the ISM. The observations indicate a range of temperatures where the higher ionization stages probe hotter gas. The hot gas fills most of the volume of the halo (scale height 3 kpc) but the volume filling factor
  • 30.
    10 The galacticecosystem in the disk is more controversial. This gas is heated and ionized through shocks driven by stellar winds from early type stars and by supernova explosions. Much of the hot, high-latitude gas may have been vented by superbubbles created by the concerted efforts of whole OB associations into the halo in the form of a galactic fountain. This hot gas cools down, “condenses” into clouds, and rains down again on the disk. In the disk, the distribution of the hot gas is quite irregular. The Sun itself is located in a hot bubble with a size of approximately 100 pc. 1.2.5 Interstellar dust The presence of dust in the interstellar medium manifests itself in various ways. Through their absorption and scattering, small dust grains give rise to a general reddening and extinction of the light from distant stars (Fig. 1.1). Moreover, polar- ization of starlight is caused by elongated large dust grains aligned in the galactic magnetic field (dichroic absorption). Furthermore, near bright stars, scattering of starlight by dust produces a reflection nebula. Finally, the interstellar medium is bright in the infrared because of continuum emission by cold dust grains. Analysis of the wavelength dependence of interstellar reddening implies a size distribution, na ∼ a−35 , which ranges from 3000 Å all the way to the molecular domain (∼5 Å). The number density of grains with sizes ∼1000 Å is 10−13 per H atom. Most of the mass of interstellar dust is thus in the larger grains but the surface area is in the smallest grains. Abundance studies in the ISM show that many of the refractory elements (e.g., C, Si, Mg, Fe, Al, Ti, Ca) are locked up in dust; i.e., dust contains about 1% by mass of the gas. Large (100 Å) interstellar dust grains are in radiative equilibrium with the interstellar radiation field at temperatures of 15 K and the absorbed stellar photons are reradiated as infrared and submillimeter continuum emission. Near bright stars, the dust temperature is higher; typically some 75 K for a compact HII region. Emission by interstellar dust dominates these wavelength regions. Rotating interstellar dust grains also give rise to emission at radio wavelengths. Very small ( 100 Å) dust grains undergo fluctuations in their temperature upon the absorption of a single UV photon and emit at mid-IR (25–60 m) wavelengths. 1.2.6 Large interstellar molecules Besides dust grains, the interstellar medium also contains a population of large molecules. These molecules are particularly “visible” at mid-IR wavelengths. The IR spectrum of most objects – HII regions, reflection nebulae, surfaces of dark clouds, diffuse interstellar clouds and cirrus clouds, galactic nuclei, the interstellar
  • 31.
    1.2 Components ofthe interstellar medium 11 Figure 1.7 An infrared view of the Eagle Nebula (image inverted) obtained with ISOCAM on the Infrared Space Observatory. The dark emission that dominates the image is due to IR fluorescence of large polycyclic aromatic hydrocarbon molecules. Near the center of the image some of the faint emission is due to large, cool dust grains present in the ionized gas. The well-known pillars are visible just under the center of this image as the three-fingered hand. Image courtesy of the ISOGAL team, especially Andrea Moneti and Frederic Schuller. medium of galaxies as a whole, and star burst galaxies – are dominated by broad infrared emission features (Fig. 1.7). These IR emission features are characteristics for polycyclic aromatic hydrocarbon (PAH) materials. These bands represent the vibrational relaxation process of FUV-pumped PAH species, containing some 50 C atoms. These species are very abundant, ∼10−7 relative to H, locking up about 10% of the elemental carbon. These PAHs may just be one – very visible – representative of the molecular Universe. In fact, visible spectra of stars generally show prominent absorption features that are too broad to be atomic in origin. These so-called diffuse interstel- lar bands (DIBs) are generally attributed to electronic absorption by moderately large molecules (10–50 C atoms). Unsaturated carbon chains, containing 10–20 C atoms, are leading candidates for these DIBs. There are now in excess of 200 DIBs known, of which some 50 are moderately strong. Because, typically, the visible spectrum of a molecular species is dominated by at most one strong transition, the DIBs implicate the presence of a large number of different molecular species. These large molecules seem to represent the extension of the interstellar grain size distribution into the molecular domain. Interstellar grains are known to contain
  • 32.
    12 The galacticecosystem several populations of nano particles: nano diamonds have been isolated from mete- orites with an isotopic composition that indicates a presolar origin; i.e., these grains predate the formation of the Solar System and they never fully equilibrated with the gas in the early solar nebula. Likewise, silicon nanoparticles may be the carrier of a widespread luminescence phenomena, the so-called extended red emission (ERE). 1.3 Energy sources 1.3.1 Radiation fields The ISM is permeated by various photon fields, which influence the physical and chemical state of the gas and dust (Fig. 1.8). The stellar radiation field contains contributions from early-type stars, which dominate the far-ultraviolet (FUV) cool stars PAHs dust CMB OB stars 10–14 10–16 10–18 10–20 10–22 10–24 10–26 10–7 10–5 10–3 10–1 101 Intensity λ(cm) hot gas Figure 1.8 The mean intensity in units of erg cm−2 s−1 Hz−1 sr−1 of the inter- stellar radiation field in the solar neighborhood. Contributions by hot gas, OB stars, older stars, large molecules (PAHs), dust, and the cosmic microwave back- ground are indicated. Figure adapted from J. Black, 1996, First Symposium on the IR Cirrus and Diffuse Interstellar Clouds, ed. R. M. Cutri and W. B. Latter (San Francisco: ASP), p. 355. The calculated X-ray/EUV emission spectrum and the FUV spectrum were kindly provided by J. Slavin. The dust emission is a fit to the COBE results for the galactic emission. The PAH spectrum was taken from ISO measurements of the mid-IR emission spectrum of the interstellar medium scaled to the measurements of the IR cirrus by IRAS (F. Boulanger, 2000, in ISO Beyond Point Sources: Studies of Extended Infrared Emission, ed. R. J. Laureijs, K. Leech, and M. F. Kessler, E. S. A.-S. P., 455, p. 3). The black squares at 12, 25, 60 and 100 m are the IRAS measurements of the IR cirrus, the DIRBE/COM measurement at 240 m, and those at 3.3, 3.5, and 4.95 m are the balloon measurement by Proneas experiment (M. Giard, J. M. Lamarre, F. Pajot, and G. Serra, 1994, A. A., 286, p. 203). Note that the latter have been superimposed on the stellar spectrum.
  • 33.
    1.3 Energy sources13 wavelengths, A-type stars, which control the visible region, and late-type stars, which are important at far-red to near-infrared wavelengths. The strength of the FUV average interstellar radiation is often expressed in terms of the Habing field, 12×10−4 erg cm−2 s−1 sr−1, named after Harm Habing, a pioneer in this field. Current estimates put the average interstellar radiation field at G0 = 17 Habing fields. Often, the radiation field in PDRs produced by a nearby star shining on a nearby cloud is expressed in terms of the equivalent one-dimensional average interstellar radiation field flux (e.g., 16×10−3 erg cm−2 s−1 ). These stellar photons are absorbed by dust grains and reradiated at longer wavelengths – in discrete emission bands in the mid-IR and in continuum emis- sion in the far-IR and submillimeter regions (Fig. 1.8). The 2.7 K cosmological background takes over at millimeter wavelengths. At extreme ultraviolet wave- lengths (EUV), even a small amount of neutral hydrogen absorbs all radiation and the intensity of the average radiation field due to stars drops precipitously at the Lyman edge (912 Å; Fig. 1.9). Stars do not contribute much at the shortest wave- lengths (X-rays). Instead, emission by hot plasmas – the coronal gas in the halo and in SNRs – dominates the radiation field. This component shows numerous emission lines. There is also an extragalactic contribution at the hardest energies. These X-ray emission components are mediated by absorption by foreground 101 10– 4 102 I λ (photons cm –2 s –1 Å –1 ) 10– 2 100 102 Interstellar radiation field OB stars Hot gas Older stars 104 106 103 λ (Å) 104 105 Figure 1.9 The average photon field in the solar neighborhood in units of photons cm−2 s−1 Å−1 . Contributions by hot gas, OB stars, and older stars are indicated. The calculated X-ray/EUV/FUV spectrum was kindly provided by J. Slavin. The visual spectrum was taken from J. S. Mathis, P. G. Mezger, and N. Panagia, 1983, A. A., 128, p. 212.
  • 34.
    14 The galacticecosystem Table 1.2 Energy balance Source P Energy density Heating rate (10−12 dyne cm−2 ) (eV cm−3 ) (erg s−1 H-atom−1 ) Thermal 0.5 60 –5 (–26)a UV — 05 5 (–26) Cosmic ray 1.0 20 3 (–27) Magnetic fields 1.0 06 2 (–27) Turbulence 0.8 15 1 (–27) 2.7 K background — 025 — a Energy loss rate. gas and, because the ionization cross section decreases rapidly with increasing energy at EUV–X-ray wavelengths, this effect is more pronounced at the longer wavelengths. Photodissociation rates and ionization rates are proportional to the photon intensity (Fig. 1.9). A simple polynomial fit for the mean photon intensity of the interstellar radiation field is ISRF = 8530×10−5 −1 −1376×10−1 −2 +5495×101 −3 cm−2 s−1 Hz−1 sr−1 (1.1) with in Å. The Habing field corresponds to about 108 photons cm−2 s−1 between 6 and 13.6 eV and about a factor of 10 fewer between 11 and 13.6 eV. 1.3.2 Magnetic fields The magnetic field is an important energy and pressure source in the ISM (cf. Table 1.2), controlling to a large extent the dynamics of the gas. The interstellar magnetic field manifests itself through linear polarization of starlight by aligned dust grains (dichroic absorption; Fig. 1.10), in polarization of far-infrared contin- uum emission of aligned dust grains, linear polarization of synchroton emission, Faraday rotation of background, polarized, radio sources, and Zeeman splitting of the 21 cm HI line and lines of molecules with unpaired electrons such as OH. The magnetic field is about 5 G in the solar neighborhood, increasing to about 8 G in the molecular ring at 4 kpc. Models for the synchroton emission, which trace the distribution of the magnetic field, imply the existence of a thin- disk component, associated with the gaseous disk, and a thick-disk component – the halo component – with a scale height of 1.5 kpc near the solar circle. The magnetic field consists of a uniform component and a non-uniform component. The uniform component of the magnetic field is roughly circular with a strength
  • 35.
    1.3 Energy sources15 Figure 1.10 The direction of the magnetic field of the Galaxy as measured through optical polarization of starlight. Upper panel: stars within 400 pc, empha- sizing the magnetic field associated with local clouds. Lower panel: stars between 2 and 4 kpc, showing some regions with magnetic field parallel to the galactic plane while others are more random. Figure reproduced with permission from D. S. Mathewson and V. L. Ford, 1970, Mem. R. A. S., 74, p. 139. of about 1.5 G in the solar neighborhood and has two field reversals within the solar circle and one outside. The field may show a spiral structure where the reversals occur in the interarm regions. There is also a considerable non-uniform magnetic field, partly associated with expanding interstellar shells (superbubbles) and their shocks. The strength of the magnetic field increases inside dense clouds, B ∼ n with 05 and typically B 30G at n 104 cm−3 . The direction of the field is correlated over the entire extent of the cloud from the diffuser outer parts to the denser cores. 1.3.3 Cosmic rays High energy ( 100 MeV nucleon−1) particles contribute considerably to the energy density of the ISM (2 eV cm−3; Table 1.2). Cosmic rays consist mainly of relativistic protons with energies between 1 and 10 GeV, 10% helium, and
  • 36.
    16 The galacticecosystem heavier elements and electrons at about the 1% level. The relative abundance of the elements in the cosmic rays is non-solar, attesting to the importance of spallation–production of light elements and an origin either in material of stel- lar (perhaps SN) composition or in sputtered interstellar grains. The interaction of energetic (1–10 GeV) cosmic-ray protons with interstellar gas gives rise to gamma rays with E 50 MeV through 0 meson decay emission. Likewise, the interaction of energetic (1 GeV) electrons with interstellar gas gives rise to gamma rays through bremsstrahlung and inverse Compton scattering. Gamma ray observations, in combination with interstellar gas surveys, can therefore be used to measure the distribution of cosmic rays in the Galaxy. Cosmic rays are tied to the galactic magnetic field and confined to a disk of radius 12 kpc with a thickness of ∼2 kpc. Cosmic rays seem to draw their energy from supernovae with an efficiency of about 10% of the kinetic energy of the ejecta. The pres- sure due to these cosmic rays provides support against gravity for the gas in the ISM. Low-energy (100 MeV) cosmic rays are important for the heating and ioniza- tion of interstellar gas. Unfortunately, their flux is difficult to measure within the heliosphere because of strong modulation by the solar wind. The measured cosmic-ray flux near the Earth is shown in Fig. 1.11 together with a fit based upon a model for the cosmic-ray injection spectrum and energy-dependence of the residence time in the interstellar medium. It is obvious that the correction is substantial at low energies. While the interstellar cosmic-ray flux is diffi- cult to measure directly, the resulting ionization drives the build-up of simple molecules (e.g., OH), which can be studied (Section 8.7). These indirect measure- ments imply a primary cosmic-ray ionization rate in the ISM of CR 2×10−16 (H atom)−1 s−1. Localized regions of much higher ionization rate may occur near associations of massive stars. 1.3.4 Kinetic energy of the ISM Winds from early-type stars and supernovae explosions supply bulk kinetic energy to the ISM. Compared to the stellar radiative budget, the total mechanical energy output is only small, ∼0.5 % (Table 1.2). However, the turbulent energy of the HI is about 6 × 1051 erg kpc−2 in the solar neighborhood and provides support for the HI gas against gravity. The HI also shows ordered vertical flow, at some 5 km s−1 in the Milky Way, as well as the infall of large gas complexes at higher latitudes. The expanding shells blown by individual stars and the superbubbles blown by the concerted action of OB associations have an important influence on the morphology of the ISM. They sweep up and compress the surrounding ISM
  • 37.
    1.3 Energy sources17 KINETIC ENERGY (MeV nucleon–1 ) PROTONS CARBON IRON a a a a b b b b (b) (a) 103 102 101 100 10–1 10–2 10–3 10–4 10–5 10–6 100 101 102 103 104 105 100 101 102 103 104 105 10–7 10–8 103 102 101 100 10–1 10–2 10–3 10–4 10–5 10–6 10–7 10–8 ALPHAS KINETIC ENERGY (MeV nucleon–1 ) DIFFERENTIAL FLUX (nucleon m –2 sr –1 s –1 MeV –1 ) a : q = 4.20, µ = 0.50 b : q = 4.10, µ = 0.60 Figure 1.11 The cosmic-ray proton flux as a function of energy measured near the Earth and the inferred interstellar cosmic-ray flux after the effects of modu- lation by the solar wind have been taken into account. Figure reproduced with permission from W.-H. Ip and W. I. Axford, 1985, A. A., 149, p. 71. and set it into motion. These motions are often unstable to Rayleigh–Taylor and Kelvin–Helmholtz instabilities and, in general, turbulence is expected to take over. This kinetic energy decays through shock waves when clouds collide, emerging as line radiation, or through the excitation of plasma waves, which, eventually, also heat the gas. The average heating due to turbulence is, however, small (Table 1.2). On a smaller scale, individual molecular clouds show linewidths in excess of the thermal width because of the presence of turbulent motions. This turbulence, which is probably of a magneto-hydrodynamic nature, supports these clouds against self-gravity. This turbulent energy is supplied by powerful outflows driven by newly formed stars into their surroundings – and hence derives from the
  • 38.
    18 The galacticecosystem gravitational energy of the collapsing cloud core – or by direct tapping of the magnetic or rotational energy supporting the cloud. 1.3.5 Summary While these energy sources are very similar in magnitude (cf. Table 1.2), how much each of them contributes to the heating of the gas (and dust) depends on the coupling processes. Understanding these processes will thus be of prime importance for understanding the physical state of the ISM. 1.4 The Milky Way Table 1.3 summarizes several characteristics of the Milky Way as a galaxy. Comparing these with those of other galaxies, the Hubble type of the Milky Way seems to be somewhere between an Sb and an Sc. In many ways, the Milky Way seems to be an average spiral galaxy. 1.4.1 Galactic distribution The molecular gas shows an exponential distribution, peaking at the molecular ring (4.5 kpc) with a scale length of about 3 kpc. In contrast, the atomic gas has a rather flat distribution out to about 18 kpc (cf. Fig. 1.12). So, within the solar circle, the surface density of the molecular gas is somewhat larger than that of the atomic gas. In contrast, the atomic gas dominates the molecular gas by far in the outer Galaxy. Within 4 kpc, there is a hole in both the atomic and molecular gas distribution except for the nuclear ring in the inner Galaxy. The total HI mass is about 5 times that of H2. The most striking aspect of the gas and dust distribution of any galaxy is the thinness of the disk. The molecular gas has a thickness of 75 pc compared with a radial scale length of 4 kpc. In the inner Milky Way, the HI scale height is two to three times larger, depending on which neutral gas component is considered. In the outer Galaxy, the neutral gas disk flares to a thickness of about 1 kpc. Nevertheless, this is still small compared to the radial scale length. The gas disk also shows a warp in the outer Galaxy. 1.4.2 Spiral structure It is difficult to discern the spiral structure of our Galaxy from the atomic gas distribution because of the presence of non-circular velocity components. Mol- ecular clouds are better tracers; not least because molecular clouds are more
  • 39.
    1.4 The MilkyWay 19 Table 1.3 The Milky Way Object Mass (M) Stars 1.8 (11) Gas 4.5 (9) Source Luminosity (L) Stellar luminosities All stars 4.0 (10) OBA 8.0 (9) Gas and dust [CII] 158 m 5.0 (7) FIR 1.7 (10) Radio 1.5 (8) -rays 3.0 (5) Mechanical luminosities SN 2.0 (8) WR 2.0 (7) OBA 1.0 (7) AGB 1.0 (4) flat rotation curve KBH rotation curve Digel R (kpc) 0 0 2 2 4 4 6 6 8 8 10 12 14 16 18 20 HI H2 Wouterloot Mass surface density (M pc –2 ) Figure 1.12 The mass surface density of HI and H2 as a function of galacto- centric radius. The distribution of HI in the outer Galaxy is quite sensitive to the adopted galactic rotation curve. Note that the nuclear ring is not shown. Figure reproduced with permission from T. M. Dame, 1993, in Back to the Galaxy, ed. S. S. Holt and F. Verter (New York: AIP), p. 267.
  • 40.
    20 The galacticecosystem limited to the arms. Long structures are present in either tracer and, for example, the Sagittarius–Carina arm is readily recognized. Nevertheless, whether the Milky Way is a two-arm or four-arm spiral has not been settled. 1.4.3 Spiral galaxies The overall distribution of the ISM can be particularly well studied in other nearby galaxies such as M31. This shows that the bright HII regions, the atomic and molecular gas, and the dust are concentrated in spiral arms. In contrast the interarm regions are much less obscured, although diffuse HI is present throughout the disk as well. Edge-on galaxies such as NGC 891 – an Sb galaxy very similar to our own – show most clearly the narrow disk component; for example, in the dust absorbing background stellar light. The HI distribution of this galaxy shows a component that is somewhat more extended than that of the Milky Way. 1.5 The mass budget of the ISM Stars of all masses in various stages of their lives indiscriminately pollute their environment with gas, dust, and metals. Except for a few elements produced in the Big Bang, the abundance of all heavy elements reflects this enrichment with stellar nucleosynthetic products. Table 1.4 summarizes gas and dust mass injection rates into the ISM for a variety of stellar objects. The dust budget has been split out into two separate columns according to whether the stellar source contains carbon-rich zones (C/O 1), which lead to carbonaceous dust formation, or oxygen-rich zones, which lead to the formation of oxides (silicates) or metals. The quoted values are uncertain – some more than others – and are often based upon various assumptions. Even estimates of the relative importance of high-mass stars versus low-mass stars evolve over time because of new developments in, for example, the determination of mass loss rates (e.g., IR studies) or nucleosynthetic reaction rates (e.g., the 12C()16O rate). Thus, while 20 years ago high-mass stars (M 8 M) were thought to be mainly responsible for the carbon in the interstellar medium, in more recent studies carbon-rich red giants dominate. These injection rates also vary across the Galaxy; i.e., because of the general increase in metallicity towards the inner Galaxy, the ratio of the O-rich to C-rich giants increases towards the galactic center, as does the Wolf–Rayet star population. The gas mass return rate is dominated by low-mass red giants, as expected, since low-mass stars dominate the stellar mass of the Galaxy. However, massive stars are more efficient in producing and injecting heavy elements, such as O and Si, in the ISM (a typical type-II supernova has an enrichment factor of ∼10 for such elements). However, on the AGB, low-mass stars inject much C formed
  • 41.
    1.5 The massbudget of the ISM 21 Table 1.4 Interstellar gas and dust budgets Source Ṁa H (M kpc−2 Myear−1 ) Ṁb c (M kpc−2 Myear−1 ) Ṁc sil (M kpc−2 Myear−1 ) C-rich giants 750 30 — O-rich giants 750 — 50 Novae 6 03 003 SN type Ia — 03d 2d OB stars 30 — — Red supergiants 20 — 02 Wolf–Rayet 100e 006f — SN type II 100 2d 10d Star formation −3000 — — Halo circulationf 7000 Infallg 150 a Total gas mass injection rate. b Carbon dust injection rate. c Silicate and metal dust injection rate. d Fraction and composition of dust formed in SN is presently unknown. These values correspond to upper limits. e Dust injection only by carbon-rich WC 8–10 stars. f Mass exchange between the disk and the halo estimated from HI in non-circular orbits and CIV studies. g Estimated infall of material from the intergalactic medium and satellite galaxies. by the triple- process. Asymptotic giant branch stars are also the site of the production of the s process (formed through slow neutron capture) elements. Type-Ia supernovae, which also have low mass progenitors, inject a considerable amount of Fe into the ISM. The time scale for stars to inject or replenish the local interstellar gas mass (8×106 M kpc−2 ) is 5×109 years. The total dust injection rate corresponds to a dust-to-gas ratio in the ejecta of ∼15%. This is somewhat larger than the average dust-to-gas ratio in the ISM, reflecting the synthesis of heavy elements in type-II supernova. Locally, star formation will convert all molecular gas into stars in only 2×108 years; the average star formation rate is heavily weighted towards the inner molecular ring. The different phases of the ISM exchange material at a rapid rate (3×107 years) and, considering all the available gas, this time scale to convert gas into stars increases to 3 × 109 years and is more comparable to the stellar injection time scale. Both rates are very short compared with the lifetime of the Galaxy (12×1010 years). The disk of the Galaxy also exchanges material with the lower halo. In particular, the concerted action of supernovae set up a galactic fountain and some 5 M is exchanged per year. In terms of the mass flux, this circulation pattern dominates the mass budget. Finally, the Galaxy is still accreting primordial material from its environment. The amount
  • 42.
    22 The galacticecosystem of this accretion is controversial. There is a contribution from satellite galaxies such as the Magellanic Clouds, and the Magellanic Stream represents an accretion of some 150 M kpc−2 Myear−1. There are also some indirect arguments that suggest that the Galaxy may have been accreting about 1 M year−1 of metal-poor primordial gas over much of its lifetime or about an order of magnitude more than the Magellanic Stream accretion. 1.6 The lifecycle of the Galaxy The origin and evolution of galaxies are closely tied to the cyclic processes in which stars eject gas and dust into the ISM, while at the same time gas and dust clouds in the ISM collapse gravitationally to form stars. The ISM is the birthplace of stars, but stars regulate the structure of the gas, and therefore influence the star formation rate. Winds from low-mass stars – and hence, the past star formation rate – control the total mass balance of interstellar gas and contribute substantially to the injection of dust, an important opacity source, and polycyclic aromatic hydrocarbon molecules (PAHs), an important heating agent of interstellar gas. High-mass stars (i.e., the present star formation rate) dominate the mechanical energy injection into the ISM, through stellar winds and supernova explosions, and thus the turbulent pressure that helps support clouds against galactic- and self- gravity. Through the formation of the hot coronal phase, massive stars regulate the thermal pressure as well. Massive stars also control the FUV photon energy budget and the cosmic-ray flux, which are important heating, ionization, and dissociation sources of the interstellar gas, and they are also the source of intermediate-mass elements that play an important role in interstellar dust. Eventually, it is the dust opacity that allows molecule formation and survival. The enhanced cooling by molecules is crucial in the onset of gravitational instability of molecular clouds. Clearly, therefore, there is a complex feedback between star formation and the ISM. And it is this feedback that determines the structure, composition, chemical evolution, and observational characteristics of the interstellar medium in the Milky Way and in other galaxies all the way back to the first stars and galaxies that formed at redshifts, z5. If we want to understand this interaction, we have to understand the fundamental physical processes that link interstellar gas to the mechanical and FUV photon energy inputs from stars. 1.7 Physics and chemistry of the ISM The key point always to keep in mind when studying the interstellar medium is that the ISM is far from being in thermodynamic equilibrium. In thermodynamic equilibrium, a medium is characterized by a single temperature, which describes
  • 43.
    1.8 Further reading23 the velocity distribution, excitation, ionization, and molecular composition of the gas. While the velocity distribution of the gas can generally be well described by a single temperature, the excitation, ionization, and molecular composition are often very different from thermodynamic equilibrium values at this temperature. This reflects the low pressure of the ISM, so that, for example, collisions cannot keep up with the fast radiative decay rates of atomic and molecular levels. Ionization and chemical composition are also kept from their equilibrium values by the presence of ∼100 MeV cosmic-ray particles – clearly a non-Maxwellian component – and a diluted, stellar, EUV-FUV photon field, which is much stronger than a 100 K medium would normally have. Finally, the large-scale velocity field is much influenced by the input of mechanical energy. Whenever a gas is not in local thermodynamic equilibrium, the level popula- tions, degree of ionization, chemical composition, and of course the temperature are set by balancing the rates of the processes involved. Much of the study of the ISM is thus concerned with identifying the various processes that control the ionization and energy balance, setting up the detailed statistical equilibrium equations and solving them for the conditions appropriate for the medium. In the remainder of this book, we will thus first study the physical and chemical processes that are of astrophysical relevance (Chapters 2–4). This is followed by a discussion of the physics and chemistry of two important components of the ISM: dust grains (Chapter 5) and large molecules (Chapter 6). With this in hand, we can examine in-depth HII regions, the phases of the ISM, photodissociation regions, and molecular clouds (Chapters 7–10). In each chapter, the ionization and thermal balance are investigated first, setting up the statistical equilibrium equations and derivingtherelevantparametersofthemedium.Thisisthenfollowedbyadiscussion of the observations of these objects and their analysis. In Chapters 11–12, non- equilibriumeffectsofadifferentnaturearestudied.Specifically,thetime-dependent effects of strong shock waves on the temperature and chemical composition of clouds are discussed in Chapter 11. Chapter 12 examines then the dynami- cal effects of expanding HII regions, supernova explosions and stellar winds. Finally, in Chapter 13, some aspects of the lifecycle of interstellar dust – which is a prime example of these types of non-equilibrium effects – are explored. 1.8 Further reading A general, technical yet accessible, introduction to the interstellar medium is provided by [1]. An introductory-level discussion of the interstellar medium can be found in [2]. A thorough but now dated monograph on physical processes in the interstellar medium is [3], which is an updated version of [4]. These two textbooks are
  • 44.
    24 The galacticecosystem no easy reading matter and the reader should beware that many a promising young scientist was last observed buying these books. A recent monograph on the interstellar medium is [5], which is intended for postgraduate courses. An unsurpassed textbook treating the physics of ionized gas is [6]. A recent overview of the properties of our Galaxy as compared to other galaxies is given by [7]. Panoramic images of the Milky Way at different wavelengths can be found on the web, http://adc.gsfc.nasa.gov/mw/milkyway.html. Each of these wavelengths traces a different component of the Galaxy. It is instructive to compare these different images. References [1] J. B. Kaler, 1997, Cosmic Clouds: Birth, Death, and Recycling in the Galaxy, (New York: Freeman Co.) [2] J. E. Dyson and D. A. Williams, 1980, Physics of the Interstellar Medium (Manchester: Manchester University Press) [3] L. Spitzer, Jr., 1978, Physical Processes in the Interstellar Medium (New York: Wiley) [4] L. Spitzer, Jr., 1968, Diffuse Matter in Space, (New York: Interscience) [5] M. A. Dopita and R. S. Sutherland, 2003, Astrophysics of the Diffuse Universe, (Berlin: Springer Verlag) [6] D. Osterbrock, 1989, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (Mill Valley: University Science Books) [7] R. C. Kennicutt, 2001, in Tetons 4: Galactic Structure, Stars and the Interstellar Medium, ed. C. E. Woodward, M. D. Bicay, and J. M. Shull (San Francisco: ASP), p. 2
  • 45.
    2 Gas cooling In thischapter, we will examine the various line cooling processes of interstellar gas. Together with the heating processes, which are discussed in the next chapter, this will allow us to solve the energy balance for the gas in a wide variety of environments. This will be applied in HII regions (Section 7.3), HI regions (Section 8.2.2), photodissociation regions (Section 9.3), and molecular clouds (Section 10.3). This chapter starts off with a refresher on the concepts of electronic, vibrational, and rotational spectroscopy (Section 2.1). We will then discuss the cooling rate (Section 2.2) with the emphasis on two-level systems (Section 2.3). A two-level system analysis is a powerful tool for understanding the details of gas emission processes and we will encounter this many times in subsequent chapters. We will then consider in detail the cooling processes in ionized (Section 2.4) and neutral (Section 2.5) gas. The last section (Section 2.6) discusses the cooling law. 2.1 Spectroscopy Table 2.1 summarizes typical properties of transitions. These are, of course, directly related to the binding energies of the species involved. Electronic binding energies for atoms increase from left to right in the periodic system from about 5 eV to some 20 eV. For hydrogen and helium the lowest electronic transitions are fairly high (10.2 and 21.3 eV, respectively); a substantial fraction of the ionization energy. Multi-electron systems have electronic orbitals that are low in energy compared with their ionization potentials. Their electronic transitions thus occur in the visible through extreme UV range. Transitions of bonding electrons in molecules typically also occur in the UV range. However, radicals or ions – which have an unpaired electron in low-lying electronic states – have transitions in the visible range. Molecular vibrational transitions involve motions of the atoms and, because of the larger reduced mass, are shifted by a factor me/M into the mid-infrared, where me and M are the masses of the electron and atom, 25
  • 46.
    26 Gas cooling Table2.1 Typical transition strengtha Type of transition ful Auls−1 Example Auls−1 Electric dipole UV 1 109 Ly 1216 Å 240×108 Optical 1 107 H 6563 Å 600×106 Vibrational 10−5 102 CO 4.67 m 34.00 Rotational 10−6 3×10−6 CSb 6.1 mm 170×10−6 Forbidden Optical (Electric quadrupole) 10−8 1 [OIII] 4363 Å 1.7 Optical (Magnetic dipole) 2×10−5 2×102 [OIII] 5007 Å 200×10−2 Far-IR fine structure 2×10−7 m 10 3m [OIII] 52 m 980×10−5 Hyperfine HI 21 cm 290×10−15 a See text for details. b The J = 1 → 0 transition. respectively. Molecules can also rotate and the centrifugal energy is proportional to 1/M. Hence, rotational energies of molecules are smaller by a factor me/M compared with electronic energies and occur at (sub)millimeter wavelengths. As an example, the binding energy of CO is 11 eV (1100 Å), the vibrational energy is 0.27 eV (2170 cm−1467m), and the rotational energy is 5×10−4 eV (2.6 mm). A final and important point to keep in mind when considering transitions between levels is that they obey certain selection rules depending on whether they are electric dipole, magnetic dipole, electric quadrupole, etc. This directly influences the strength of the transitions (Table 2.1 and Section 2.1.4). We will come back to this later (Table 2.2 and Section 2.1.4). 2.1.1 Electronic spectroscopy Atoms Each atomic line can be related to a transition between two atomic states. These states are identified by their quantum numbers. The states are characterized by the principal quantum number, n (1, 2, …); the orbital angular momentum, (which can have values between 0 and n−1); and the electron spin angular momentum, s±1/2. The orbital angular momenta are designated by s, p, d, … , corresponding to = 0 1 2, … The total angular momentum, j, is the vector sum of and s. For hydrogen, neglecting fine structure, the energies of the atomic states depend only on n. Hydrogen transitions can be grouped into series (downward transitions, m → n, or upward transitions, n → m) according to the value of n, with the conventional names for the first few: Lyman (n = 1), Balmer (n = 2), Paschen
  • 47.
    2.1 Spectroscopy 27 Table2.2 Selection rulesa Electric dipole Magnetic dipole Electric quadrupole “allowed” “forbidden” “forbidden” 1 J = 0±1 J = 0±1 J = 0±1±2 0 0 0 0 0 0, 1/2 1/2, 0 1 2 M = 0±1 M = 0±1 M = 0±1±2 0 0 when J = 0 0 0 when J = 0 3 Parity change No parity change No parity change 4b One electron jumping For all electrons One electron jumping with l = ±1n arbitrary l = 0n = 0 l = 0±2n arbitrary or for all electrons l = 0n = 0 5c S = 0 S = 0 S = 0 6c L = 0±1 L = 0J = ±1 L = 0±1±2 0 0 0 00 1 a A short summary of six selection rules describing electric dipole and quadrupole and magnetic dipole transitions. b With negligible configuration interaction. c For LS coupling only. Hydrogen energy levels (a) 1 1 s 3 2 p d f –1 –3 –5 –7 –9 –11 –13 E (eV) (b) Hydrogen energy levels 1 1 s 3 2 p d f –1 –3 –5 –7 –9 –11 –13 – 15 E (eV) Figure 2.1 The energy level diagram of hydrogen. The principal quantum number, n, increases upwards (for clarity only the lower ones are labeled). The orbital angular momentum number, , increases rightwards and only the first few are shown. They are labeled s, p, d, f. The arrows indicate some allowed transi- tions. (a) The Lyman through transitions are indicated. (b) The transitions between the lowest levels are indicated. Note that the 2s level is metastable. (n = 3), Brackett (n = 4), Pfund (n = 5), and Humphreys (n = 6). Successive lines in these series are labeled , etc. Thus, Lyman emission corresponds to the transition n = 2 → 1. The Balmer transitions are also indicated by the designation, H, H, etc. For heavier species, the electrons are grouped in closed and open shells. The shell energies depend mainly on n. Subshells within each shell have different
  • 48.
    28 Gas cooling angularmomenta and are labeled by n, and the number of electrons in the subshell (to the maximum possible 22+1). For heavier species, the description of states involved in transitions becomes more complicated than for H because of the interaction between the various electrons present. The energy levels of an atom are characterized by the values of J, the magnitude of the total angular momentum. The total angular momentum, J is defined by J = L+S. Here, L is the orbital angular momentum: the vector sum of the orbital angular momenta of the individual electrons. The spin of the atom, S, is the sum of the electron spins, where each electron has a spin 1/2 that can be parallel or antiparallel. The total angular momentum, J, is formed by vector addition and can take the integral values, L+SL+S−1L+S−2L−S. There are thus 2S+1 possible J values. When relativistic effects are small, a level with a given L and S is split into a number of distinct levels with different values of J, because of spin-orbit coupling. This coupling is a result of the interaction of the magnetic moment of the electron spin with the magnetic moment of the orbital motion. This fine-structure splitting is a small perturbation – of the order of 2 Z4 , with the fine-structure constant = 1/137 and Z the nuclear charge. Transitions among the fine-structure levels occur therefore at near-to far-IR wavelengths. Energy levels are then designated by term symbols, 2S+1 LJ , with the different values for the total orbital angular momentum, L = 012 , denoted by capital roman letters, S, P, D, …1 The multiplicity of the term, 2S +1, can take values 1, 2, 3, … , and leads to the designation singlet, doublet, triplet, … , levels. Each of these levels is still degenerate with respect to the orientation of J; e.g., J is space quantized in a magnetic field. Designating M as the component of J along the direction of the magnetic field, M can take the values, JJ −1J −2−J (e.g., gJ = 2J +1). Generally, the nuclear spin is of little concern. However, for HI, the hyperfine structure, resulting from the interaction of the magnetic moments of the electron and the nucleus, is of particular interest because it gives rise to the 21 cm line. This interaction is quite weak and the resulting splitting is very small. All this terminology of electronic terms can be interpreted in classical terms of orbital and spin angular momentum and the coupling between them. However, in addition, there is the quantum-mechanical concept of parity, which has no classical analog. Parity describes the behavior of the wave function, xyz, under reflection where even parity corresponds to xyz = −x−y−z and odd parity, xyz = −−x−y−z. Odd parity is indicated by a left superscript o. Selection rules for electronic transitions are summarized in Table 2.2. These merely reflect conservation principles. Thus, angular momentum has to be 1 Unfortunately, convention uses very similar notation for L = 0 (S) and for the electron spin angular momen- tum (S).
  • 49.
    2.1 Spectroscopy 29 conservedunder vector addition and, given that the photon has one unit of angular momentum, this leads to the J and L rules. The electron spin can only be changed by a magnetic field. The parity rule reflects that the dipole operator in the transition moment integral (proportional to the transition probability) has odd parity and hence only couples states with opposing parity. In the weak spin- orbit coupling regime (LS-coupling, also called Russell–Saunders coupling), the fine-structure energies are small compared to the energy differences of the levels. In the LS-coupling case, both L and S are “good” quantum numbers, which are conserved. When the coupling increases, (e.g., for heavier elements with larger nuclear charge), this rule breaks down and the spin-orbit interactions become as strong as the interactions between individual spins or orbital angular momenta. For LS coupling, conservation of S results in the segregation of transitions into groups associated with singlet, triplet, or quintet states, and transitions between these different S-states are weak. Conversely, this then implies that a species can become “trapped” in the lowest triplet or quintet state, which can represent considerable internal excitation. As an example, consider neutral carbon with six electrons (1s2 2s2 2p2 ). In the ground state, four of these are paired up in 1s and 2s orbitals and two occupy 2p orbitals but with the same spin (e.g., 1s22s22p2 where the superscript indicates the number of electrons in the shell). These parallel spins give favorable exchange interactions. The ground state has thus L = 1S = 1, and J = 0, e.g., 3P0. Fine- structure interaction then produces two levels, 3P1 and 3P2 at 16.4 and 43.4 cm−1 above the ground state. The state where the electrons have opposing spin has slightly higher energy and has L = 2S = 0, and J = 2 with designation, 1 D2 at about 10 000 cm−1 above ground. The next state up is a 1 S0 state at about 21 600 cm−1 . These levels arise from the same electron configuration and are thus only linked through forbidden transitions. In particular, the 3 P levels are connected through fine-structure transitions at 609 and 370 m. The forbidden character of transitions between the low-lying states is a very general point: all the low-lying electronic levels of the more common species arise from the same electronic configuration as their ground state and transitions are forbidden because of the parity selection rule. Molecules We will first discuss diatomic molecules, which in many ways we can describe in terms of an equivalent atom. Many of the concepts of atomic spectroscopy carry directly over into molecular spectroscopy. The nuclei in diatomic molecules produce an electric field along the internuclear axis. The component of the angular momentum vector of each electron along the internuclear axis can only take on values ml = ll − 1l − 2−l. The variable, , is then defined
  • 50.
    30 Gas cooling asthe absolute value of ml, where = 012 correspond to completely analogous to atomic s, p, d, orbitals. For centrosymmetric species (e.g., homonuclear diatomics), parity under inversion is indicated by g for even and u for odd (the wave function does not change sign and changes sign, respec- tively). The term symbols of electronic levels in diatomic molecules are then characterized by the component of the total angular momentum along the inter- molecular axis, equal to the sum of the s. Analogous to the designation, S, P, D, for atoms, we have for diatomic molecules, corre- sponding to = 012 The multiplicity of the term is indicated by a super- script with the value 2S + 1, reflecting the orientation of the total spin angular momentum, , with respect to the internuclear axis, where can take the values SS − 1S − 2−S.2 The total angular momentum is the sum of the orbital and spin angular momenta along the internuclear axis, = + . We can then designate molecular terms as 2S+1ug, where the subscripts only apply to centrosymmetric species. For terms, a ± superscript indicates the behavior of the wave function under reflection in the plane containing the two nuclei. As for atoms, coupling of the magnetic moments associated with spinning/orbiting charges is important. In this case, in addition to the orbital and spin angular momenta, we also need to consider the nuclear rotations. Two limiting cases are of interest: Hund’s case (a), where the latter is unimportant and spin–orbit coupling is dominant, and Hund’s case (b), where the spin couples strongly to the nuclear rotations and weakly to the orbital motions. As for atoms, the total angular momentum must change by 0 or 1 (and J = 0 0). The selection rules for allowed electronic transitions are now = 0±1S = 0 = 0, and = 0±1. The selection rules associ- ated with symmetry are now: for terms, only + ↔ + and − ↔ − are allowed. For centrosymmetric species, only transitions with a change in parity (e.g., u ↔ g) are allowed. As for atoms, the conservation of electronic spin can lead to metastable excited levels. So, molecular hydrogen has the ground state 1 + g with S = 0, = 0, and = 0 and bound excited singlet states, 1 + u and 1 u (Fig. 2.2). There is also a repulsive electronic triplet state, 3 + u , at intermediate energies and bound excited triplet states (3u and 3 + g ). Molecular oxygen has a ground state 3 − g and carbon monoxide has the ground state 1 +. Singlet–triplet transitions are forbidden and the first allowed electronic transitions for molecular hydrogen are the Lyman (1 + g →1 + u ) and Werner (1 + g →1 u) bands. In polyatomic molecules, electronic transitions are often connected to the exci- tation of specific types of electrons or electrons associated with a small group of 2 Again, convention uses very similar notations for the = 0 term () and the total spin angular momentum ().
  • 51.
    2.1 Spectroscopy 31 H(1s)+p+e 145792 H(1s)+H(2s,2p) 118 372 H(1s)+H(1s) 36 113 cm–1 cm–1 124 429 1.06 99 143 90 203 1.03 1.29 14.671 18.071 v = 14 v = 10 v = 5 eV 4.476 v = 0 0.7416 –2179 0 1 2 Å 800 Å 850 Å 100 Å 1108 Å H+ 2 1Σ+ u 1 Σ+ g 2 Σ+ g 3Σ+ g 1 Πu 3Πu 3Σu + 1 Σg + Figure 2.2 Schematic of the low-lying electronic energy levels of H2 as a function of the separation between the two hydrogen atoms. Some vibrational levels in the ground electronic state are also schematically included. Dissoci- ation and ionization energies are indicated on the right-hand side in eV and cm−1 . Some excitation energies are given on the left-hand side in cm−1 . All energies are relative to the v = 0 level in the ground electronic state. Figure reproduced from G. B. Field, W. B. Somerville, and K. Dressler, and reprinted with permission from Ann. Rev. Astron. Astrophys., 4, p. 207, ©1966, by Ann. Rev. (www.annualreviews.org). atoms in the species. For example, the ← transition in a C= C bond occurs around 60 000 cm−1 (1600 Å), while in a C= O bond this transition occurs around 36 000 cm−1 (2800 Å). In a conjugated chain, this transition will shift to longer wavelengths. It is not possible to review the spectroscopy of polyatomic species here. In short, for a given nuclear geometry, molecular orbitals are constructed from atomic orbitals. The available electrons are then divided over these orbitals to construct electronic configurations, taking electron correlation into account (e.g., the Pauli principle). The lowest electronic configuration is then the ground state. For organic molecules, the ground state possesses a “closed shell” configu- ration (i.e., all the bonding and non-bonding orbitals are filled with a pair of elec- trons). Radicals (and often ions) possess molecular orbitals with only one electron. For our purposes, the relevant electrons are located in the highest filled molec- ular orbitals. Deeper-lying electrons are not easily excited by the relatively low energies of photons in the interstellar medium. Radicals have transitions at low energies (i.e., in the visible range of the spectrum) associated with the partially occupied molecular orbital. Ground-state configurations for species where the
  • 52.
    32 Gas cooling electronsare paired must be singlet and are labeled S0 where the S indicates singlet and the 0 the ground state. Electronically excited singlet states are then enumerated according to their energy as S1S2 Excited states do not have to have paired electron spins. Hence, we can distinguish triplet states, which are designated T1T2 (the subscript 0 is exclusively assigned to the ground state). The lowest triplet state is at somewhat lower energy than the first excited singlet state, reflecting the better electron correlation in the former. This energy differ- ence ranges from ∼03 to 3 eV. For polycyclic aromatic hydrocarbons this energy difference is typically 1–1.5 eV (Fig. 2.3). As for atoms and diatomic molecules, allowed radiative transitions have no change in electron spin. Hence, the lowest triplet state is metastable with respect to the ground state. Up to now, we have purposely avoided the most striking aspect of molecular spectra, the presence of considerable substructure associated with nuclear vibra- tions and rotations. The quantization of vibrations and rotations will be discussed in more detail in Sections 2.1.2 and 2.1.3. What is of importance here is that during Figure 2.3 Schematic diagram of singlet and triplet electronic levels of an organic molecule. The electronic states are displaced horizontally for clarity but this has no physical meaning. The vertical axis denotes energy on an arbitrary scale. Within each electronic state, vibrational levels are schematically displayed. The rotational sublevels are not shown.
  • 53.
    2.1 Spectroscopy 33 anelectronic transition, the molecule may also change its vibrational or rotational state. This may result in vibrational sidebands, which are shifted in energy by one or more quanta of vibrational energy (∼1000–3000 cm−1 ). In addition, these bands also show small-scale structure caused by simultaneous rotational transi- tions. As for vibrations, these are classified in terms of classical P, Q, and R branches (cf. Section 2.1.2). The general appearance of these bands reflects the difference in rotational constants between the two electronic states involved in the transition. 2.1.2 Vibrational spectroscopy The essence of vibrational spectroscopy is contained within Hooke’s law for a harmonic oscillator, = 1 2 c (2.1) with the fundamental frequency, the force constant, and the reduced mass of the molecular units vibrating. The factor c is included to transform the unit to wavenumbers (cm−1) for cgs units, commonly used in spectroscopy. Molecular bond strengths are, of course, very similar to binding energies of electrons to an atom. However, the frequencies of the transitions of molecular vibrational levels are, thus, shifted to lower energies by me/M, with me and M the masses of the electron and atom, respectively, and occur in the near- and mid-IR. Real molecules are not harmonic oscillators and, as a result of anharmonicity of the bonding, hot bands (e.g., 2–1, 3–2) are generally shifted to lower frequencies. All the possible vibrational motions of a molecule can now be described in terms of these fundamental modes. A non-linear molecule containing N atoms will have 3N − 6 normal modes; a linear molecule 3N − 5. Not all of these modes will lead to distinct absorptions. Some of the modes will occur at the same frequency and hence these modes are degenerate. Others will not be infrared-active because the dipole moment does not change during the vibration. So, as an example, methane has nine fundamental modes but only the symmetric and asymmetric stretching and bending vibrations of the C–H bonds about the central C atom show up in the IR absorption spectrum: the 41306cm−1 , the 21534cm−1 , and the 33019cm−1 modes. Vibrations in homonuclear molecules do not lead to changes in the dipole moment and hence are inactive in the infrared. Mixing of isotopes in such species will lead to infrared absorptions. Likewise, interactions with the environment in a solid will introduce weak infrared activity. Vibrational transitions are very characteristic for the motions of the atoms in the molecular group directly involved but much less sensitive to the structure of the rest of the molecule. Characteristic band positions of various molecular groups
  • 54.
    34 Gas cooling X–Hbond stretching modes C C C C C C C C N O O O O O O O O C C C C C C H H H H H H H H H H O C C C C C C C C S N H H O N 4000 cm–1 3500 3000 2500 2000 1500 1000 500 20 15 10 9 8 7 6 5 4 Wavelength (µm) 3 2.5 µm H H H X–X bond stretching modes bond bending modes C Figure 2.4 Summary of the vibrational frequencies of various molecular groups. The filled boxes indicate the range over which specific molecular groups absorb. The vibrations of these groups are schematically indicated in the linked boxes. Figure kindly provided by D. Hudgins. are illustrated in Fig. 2.4 and summarized in Table 2.3. Modes involving motions of hydrogen occur at considerably higher frequencies than modes involving similar motions of heavier atoms (Hooke’s law again). Thus, H-stretching vibrations occur in the 3 m region, while stretching motions among (single-bonded) C, N, and O atoms are located around 10 m. Likewise, when the bond strength increases, the vibration shifts to higher frequencies. Simplistically speaking, single bonds are characterized by two electrons occupying the bonding molecular orbital. Similarly, double bonds and triple bonds correspond to four and six electrons in the bonding molecular orbitals. Because the bond strength increases from single bonds to double bonds to triple bonds, the location of singly, doubly, and triply bonded CC vibrations shifts from about 1000 to 2000 cm−1 (Fig. 2.4). The actual spectrum of a gaseous compound is much more complex because transitions are actually combined rotational–vibrational transitions where both the vibrational and the rotational energy can change. This gives rise to three sets of absorption bands corresponding to J = 1 (R branch), J = 0 (Q branch), and J = −1 (P branch), where J = J −J with J and J the rotational quan- tum numbers in the upper and lower vibrational state, respectively. For elec- tric quadrupole transitions, the selection rules are J = 2 (S branch), J = 0 (Q branch), and J = −2 (O branch). These sets of bands are, of course, shifted from each other by the difference in rotational energy content of the species. Consider as an example a diatomic molecule, which we represent here as a harmonic oscillator and a rigid rotor. The energy is then given by EvJ = v+ 1 2 +B J J +1 (2.2) with v and J the vibrational and rotational quantum number, the vibrational frequency, and B the rotational constant. The R branch (v = 1 → 2 J → J + 1)
  • 55.
    Exploring the Varietyof Random Documents with Different Content
  • 56.
    calm and exalted.Her blood ran light. Having destroyed her world, her disbelief somehow survived as if on an eminence. However, her emotions rejected their own finality. She felt that she had to go on somewhere outside herself. May waited in vain for Paul to come back. She convinced herself that she was not good. When she believed in her own humility she was not afraid to admit that she wanted to see him. She was unhappy now with her own body. As soon as she saw her little breasts uncovered she felt frightened and ashamed and wanted to hide herself. When she was alone in her room she cried miserably, but as soon as her tears ceased to flow she lay on her bed in an empty waiting happiness, thinking of Paul. She recalled all that related to him since she had first known him. It gave her a beautiful happy sense of want to remember him so distinctly. However, when her thoughts arrived at the memory of the last thing that had occurred between them she imagined that she wished him to kill her so that she need no longer be ashamed. I want to be dead! I want to be dead! She said this over and over into her pillow. Her beautiful pale braid of hair was in disorder. Her thin legs protruded from her wrinkled skirts. She lifted her small tear-smudged face with her eyes tight shut. May wanted to tell Aunt Julia, but dared not. She knew Aunt Julia was sad, though she did not know why. Aunt Julia, however, resisted confidences. When she came in from work and found May waiting for her in the hall or on the stairs Aunt Julia made herself look tired and kind. Well, May, dear, how are you? You seem to be a very bored young lady these days. Your father is thinking of sending you away to school when Bobby goes. How would you like that? And she smiled in a perfunctory far-away fashion.
  • 57.
    May saw thatAunt Julia was in another world and did not want her. I don't care. Whatever you and Papa decide. I'm an awful ninny and should be terribly homesick. That would be good for you. You must learn to be self-reliant. Without glancing behind her, Aunt Julia passed quickly up the stairs and disappeared into her room. The door shut. To May it was as if Aunt Julia knew everything already and put her aside because of what she had done. She was dead and corroded with shame. Lonely, she wandered out into the back yard. The sky, in the late sunshine, was covered with a pale haze like faint blue dust. A shining wind blew May's hair about her face and swirled the long stems of uncut grass. The seeded tops were like brown-violet feathers. Beyond the roofs and fences the horizon towered, vast and cold looking. May wanted it to be night so that she could hide herself. She knew Nellie was in the kitchen doorway watching her. She wanted to avoid the eyes of the old woman. Paul could not love her while she was despised. White clothes on a line were stretched between the windows of the apartment houses that overhung the alley. The bleached garments, soaked with blue shadow, made a thick flapping sound as the wind jerked them about. When the sun sank the grass was an ache of green in the empty twilight. May thought it was like a painful dream coming out of the earth. She was afraid of the fixity of the white sky that stared at her like a madness. She knew herself small and ugly when she wanted to feel beautiful. If she were only like Aunt Julia she would not be ashamed. It grew dark. She loved the dark. There was a black glow through the branches of the elm tree against the fence. The large stars, unfolding like flowers, were warm and strange. In the enormous evening only a little shiver of self-awareness was left to her. She tried to imagine that, because she was ugly and impure, Paul had
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    already killed her.The strangeness and exaltation she felt came to her because she was dead. She loved him for destroying her. Dudley gave up the attempt to take Laurence into his life. Dudley had insisted on seeing the Farleys several times, but the result of these meetings was always disappointing. What he considered their small hard pride erected about them a wall of impenetrable reserves. He pitied them in their conventionality. They regard me, he thought, as a wrecker of homes, and the fact that I have been Julia's lover prevents them from recognizing me in any other guise. He felt that he was learning a lesson. He must avoid destructive intimacies. If he gave, even to small souls, he had to give everything. In order to save himself for his art he must learn to refuse. He was in terror of love, in terror of his own necessities, and afraid of meeting acquaintances who, with the brutality of casual minds, could shake his confidence in himself by uncomprehending statements regarding his work. He grew morbid, shut himself up in his studio, and refused to admit any validity in the art of painters of his own generation. He persuaded himself that he was the successor of El Greco and that since El Greco no painter had done anything which could be considered of significance to the human race. He would not even admit that Cézanne (whom he had formerly admired) was a man of the first order. He was a painter, to be sure, but Dudley could ally himself only with those whose gifts were prophetic. His imaginings about himself assumed such grandiose proportions that he scarcely dared to believe in them. To avoid any responsibility for his conception of himself he was persuaded that there was a taint of madness in him. Rather than awaken from a dream and find everything a delusion, he would take his own life. He lay all day in his room and kept the blinds drawn, and was tortured with pessimistic thoughts, until, by the very blankness of his misery, he
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    was able toovercome the critical conclusions of his intelligence. He did not eat enough and his health began to suffer. His absorption in death drew him to concrete visions of what would follow his suicide. He was unable to close his eyes without confronting the vision of his own putrid disintegrating flesh. In his body he found infinite pathos. As much as he wanted to escape his physical self, it was sickening to think of leaving it to the indignities of burial at the hands of its enemies. The idea of suicide, haunting him persistently, aroused a resistant spirit in him. He exaggerated the envies of his contemporaries. He fancied that they feared him far more than they actually did and were longing for his annihilation. He decided that something occult which originated outside him was impelling him toward self- destruction. In refusing to kill himself he was combating evil suggestions rather than succumbing to his own repugnance to suffering and ugliness. While he was in this frame of mind some one sent him a German paper that was the organ of an obscure artistic group. In this journal, insignificantly printed, was a flattering reference to Dudley. He was called one of the leaders of a new movement in America. He read the article twice and was ashamed of the elation it afforded him. He could not admit his deep satisfaction in such a remote triumph. With a sense of release, he indulged to the full the vindictiveness of his emotions toward his own countrymen—those who were fond of dismissing him as merely one of the younger painters of misguided promise. However, the praise from men as unrecognized as himself encouraged his defiance to such a point that he resumed work on a canvas which he had thrown aside. His own efforts intoxicated him. He refused to doubt himself. Life once more had the inevitability of sleep. He knew that he was living in a dream and only asked that he should not be disturbed. He needed to run away from the suggestion of familiar things. He decided to go abroad again and wrote to borrow money of his father.
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    Dudley made uphis mind to avoid Paris where, as he expressed it, the professional artist was rampant. He wanted to visit the birthplace of a Huguenot ancestor who had suffered martyrdom for his religion. It stimulated him to think of himself as the last of a line whose representatives had, from time to time, been crucified for their beliefs. Two endless streams of people moved, particolored, in opposite directions along the narrow street. The high stone buildings were tinged with the red of the low sunshine. Hundreds of windows, far up, catching the glare, twinkled with the harsh fixity of gorgon's eyes. Beyond everything floated the pale brilliant September sky overcast by the broad rays which stretched upward from the invisible sun. Julia, returning from the laboratory, hesitated at a crowded corner and found Dudley beside her. This is pleasant, Julia. I've been wanting to see you and Laurence Farley. I'm sailing for Europe next week, and I should have been very much disappointed if I had been obliged to go off without meeting you again. He tried to speak easily while he looked at her with an expression of reproach. Julia smiled and held out her hand. There was a defensive light in her eyes which he interpreted as a symptom of dislike. He wanted to convince himself that every one, even she, was completely alienated from him. All that fed his pain strengthened his vacillating egotism. Julia noted the familiar details of his appearance: his short arms in the sleeves of a perfectly fitting coat; the plump hairy white hand which reached to hers a trifle unsteadily; his short well-made little body that he held absurdly erect; the wide felt hat that he tried to wear carelessly, which, in consequence, was slightly to one side on the back of his head and showed his dark curls; the childishly fresh color which glowed through the beard in his carefully shaven cheeks;
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    his small fullmouth that sulked in repose but when he smiled displayed exaggeratedly all of his little even teeth; his prettily modeled, womanish nose; the silky reddish mustache on his short lip; and his soft, ingratiating, long-lashed eyes. Everything in his appearance disarmed her resentment of him. Yet she knew that if she expressed anything of her state of mind he would take advantage of her vulnerability. She was prepared to see his gaze harden toward her and his demeanor, puerile now, become ruthless and commanding. She could not analyze the thing in herself that made her so helpless before him. She was able, she thought, to observe him coldly. She withdrew her hand from his and said, So you are going away again? I am glad for your sake. I know how America must irk you. Even from my viewpoint I can see that it is the last country for an artist. At the same moment her heart contracted and she told herself that there was something false and monstrous in Dudley which suppressed her natural impulse to be frank in stating what she felt for him. Dudley walked beside her. She wants me to go away! He insisted on believing this. To know that she continued to suffer, however, comforted him as much now as it had in the past. He sensed that she had, in some remote way, remained subject to him. Because of this she was dear. When he remembered that, but for this accidental meeting, he would not have communicated his departure to her he was momentarily panic-stricken. He no longer wished to detach himself from her. Tell me about your work. What are you doing now? He took her arm. I can't talk about my work, Julia. Something goes out of me that ought to go into the work when I talk about it too much. That's my struggle—my fight. It's terrifying at times. I know all the hounds are baying at my heels. When I go abroad this time I am going to avoid Paris. I know dozens of cities. Paris is the only one which is a work of art. That's why I am going to keep away. I am through with the finality of that kind of art. I am going abroad to feel how much of an American I am. That's why I hate it so. It's in
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    me—a part ofme. I can't escape it. I must express it. That is my salvation—in belonging to America. It was almost irresistible to tell her some of the conclusions he had arrived at to comfort himself, but he knew that Julia never approached a subject from a cosmic angle. She made him feel small and unhappy and full of a homesickness for understanding. In her very crudity she was the life he had to face. I want to talk to you about yourself, Julia. There are clouds of misunderstanding between us. We mustn't leave things like this. He pressed her arm against his side. She was ashamed before a stout woman who was passing who showed, by the expression of dull attention in her eyes, that she had overheard his remark. In this atmosphere of public intimacy Julia felt grotesque. I can't talk about myself, Dudley. Don't ask me. You've put me out of your life. Why should you be interested? He was conscious of the stiffening of her body as she walked beside him and observed the forced immobility of her face. Emerging from the self-loathing which was an undercurrent to his vanity, he was grateful to her for allowing him to hurt her. He began to wonder if he were not, at this instant, realizing for the first time the significance of his relationship to her—not its significance in her life, but its significance in his own. He admitted to himself the cruelty of his feeling for her. He wanted to torture her, to annihilate her even. It pleased him to discover in himself enormous capacities for all things that, to the timid-minded, constitute sin. He must embrace life without moral limitations. Julia, my dear—you must not misunderstand my feeling for you. I want you—want you even physically—as much as I ever did. His voice shook a little. It is only because I understand now that I must refuse myself much. I have found just this last month a marvelous spiritual rest which makes living deeply more acceptable. Julia had never felt more contemptuous of him. What I have to say would only convince you of my limitations. Don't be childish, Julia. You don't want to understand me. We can't talk in the street. Come to my studio for half an hour. He could not
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    let her goaway from him yet. Julia's pride would not allow her to object. On the way they passed an acquaintance of Dudley's. Dudley could not explain to himself why he was ashamed of being seen with Julia. He wanted to hurry her through the street. In the oncoming twilight the brilliant shop fronts were vague with glitter and color. Above the glowering tower of an office building a blanched star twinkled among faded clouds. When they reached Dudley's doorstep Julia began to feel morally ill and to wonder why she had come. As Dudley watched her mount the long green- carpeted stairs before him he was suddenly afraid of her. They entered the studio. It was almost dark in the big room. The canvas that Dudley was working on stood out conspicuously in the translucent gloom that filtered through the skylight. He crossed the floor and furtively threw an old dressing gown over the painting. Julia found herself unable to speak. When she discerned the lounge she sat down weakly upon it. Dudley stumbled over the furniture. He wanted to evade the moment when he must find the lamp. Take off your wrap, Julia. I can't find matches. I seem to have mislaid everything. I am a graceless host. His own voice sounded strange to him. When at last he struck a match, Julia said, Don't! and put her hands to her eyes. The flame, which, for an instant, had blindly illumined his face, went out. Dudley could not bring himself to move. The evening sky, dim with color, was visible through the windows behind him, and above the sombre roof of the factory that rose from the courtyard his figure was thrown into relief. Objects over which there seemed to brood a peculiar stillness loomed about the room. The tension was intolerable to them both. They were experiencing the same nausea and disgust of their emotions—emotions which seemed inevitable for such a moment and so meaningless. Dudley said, Where are you? I'm afraid of stumbling over you.
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    Julia, a hystericalnote in her voice, answered, Here I am, Dudley. She knew that he was coming toward her. She wanted to die to escape the thing in herself which would yield to him. But at this instant the light flashed on and everything that she was feeling appeared to her as unjustifiable and ridiculous. To Dudley, Julia's body represented all the darkness of self-distrust and the coldness of his own worldly mind. He wished that her personality were more bizarre so that he might regard his past acts as mad rather than commonplace. He did not know why he had brought her to the studio and was ashamed to look at her. There was nothing for it but to admit the duality of his nature, and that half of it was weak. He longed to hasten the time of sailing when he would begin completely his life alone in which nothing but the artist in him would be permitted to survive. He said, Is it too late for me to make you some tea? Let me take your wrap. When he approached her he averted his gaze. I can't stay long, Dudley. It is better that I shouldn't. She wanted to force on him an admission of her defeat. If she could only reproach him by showing him the destruction of her self-respect! Her eyes were purposely open to him. He would not see her. She resented his obliviousness. You seem to me a master of evasion. When he sat down near her, he said, Let it suffice, Julia, that I take the hard things you want to say to me as coming from a human being whom I respect and care for enormously—and I still think everything fine possible between us provided you accept in me what I have never doubted in you—my absolute good faith, and my absolute desire, to the best of my powers, to be honest and sincere in every moment of our relationship, past and present. Julia gave him a long look which he obliged himself to meet. Then she got up. I can't stay, Dudley. You won't understand. She turned her head aside. Her voice trembled. It's painful to me. He rose also, helplessly. He wanted to wring a last response from her. It was impossible. Everything seemed dark. He would not
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    forgive her forgoing away. Julia took up her wrap from a chair and went out hastily without looking back. Dudley felt a swift pang of despair. Not because she was gone, but because her going left him again with the problem of reviving the hallucinations of greatness. It was not easy for him to deceive himself. He could do so only in the throes of emotions which exhausted him. In moments of unusual detachment he perceived the faults in himself as apart from the real elements of genius that existed in his work. But he was not strong enough to continue his efforts for the sake of an imperfect loveliness. Only in spiritual drunkenness could he conquer his susceptibility to the nihilistic suggestions of complacent and unimaginative beings. PART III Julia and Laurence were to dine with Mr. and Mrs. Hurst. Of late Laurence had shown an unusual measure of social punctiliousness. Julia realized that his new determination to see and be with people was a part of his resistance to suffering. She thought bitterly that his regard for the opinions of others was greater than his regard for her. Julia put on a thin summer gown, very simply made, a light green sash, and a large black hat. Her misery had pride in itself, but when she looked in the glass she was pleased, and it was difficult to preserve the purity of her unhappiness. As she descended the stairs at Laurence's side she felt guiltily the trivial effect of her becoming dress. She wanted him to notice her. I'm afraid we are late. His fine eyes, with their sharp far-away expression, rested on her without seeming to take cognizance of her. I hope not. Mrs. Hurst is a hostess who demands punctuality. He spoke to her as to a child. There was something cruel in his kindness. For fear of exposing himself he refused her equality.
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    If he wouldonly love her—that is to say, desire her—Julia knew that she would be willing to make herself even more abject than she had been, and that it would hurt her less than his considerate obliviousness. Laurence had ordered a taxi-cab. The driver waited at the curbstone in the twilight. He turned to open the door for the two as they came out. Julia was avidly, yet resentfully, aware of his surreptitious admiration. She told herself that her sex was so beggared that she accepted without pride its recognition by a strange menial. It was a beautiful cool evening. The glass in the taxi-cab was down. The cold stale smell of the city, blowing in their faces, was mingled with the perfume of the fading flowers in the park through which they passed. The trees rose strangely from the long dim drives. Here and there lights, surrounded by trembling auras, burst from the foliage. Far off were tall illuminated buildings, and, about them, in the deep sky, the reflection was like a glowing silence. The wall of buildings had the appearance of retreating continually while the cab approached, as if the huge blank bulks of hotels and apartment houses, withdrawing, held an escaping mystery. Laurence scarcely spoke. Julia's sick nerves responded, with a feeling of expectation, to the vagueness of her surroundings. Her heart, beating terrifically in her breast, seemed to exist apart from her, unaffected by her depression and fatigue. It was too alive. She cried inwardly for mercy from it. Mrs. Hurst's home was a narrow, semi-detached house with a brown-stone front and a bow window. From the upper floor it had a view of the park. When Julia and Laurence arrived, a limousine and Mr. Hurst's racer were already drawn up before the place. There were lights in one of the rooms at the right, and, between the heavy hangings that shrouded its windows, one had glimpses of figures. Laurence said sneeringly, Hurst has arrived, hasn't he! Affluent simplicity in a brown-stone front. You are honored that Mrs. Hurst is carrying you to glory with her.
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    Julia said, Butthey really are quite helpless with their money, Laurence. Mrs. Hurst has a genuine instinct for something better. How ceremonious is this occasion anyway? I don't know whether I am equal to the frame of mind that should accompany evening dress. There will only be one or two people. Mrs. Hurst knows how we dislike formal parties. Mr. Hurst, waving the servant back, opened the front door himself. He was a tall, narrow-shouldered man with a thin florid face. His pale humorous blue eyes had a furtive expression of defense. His mouth was thin and weak. His manner suggested a mixture of braggadocio and self-distrust. He dressed very expensively and correctly, but there was that in his air which somehow deprecated the success of his appearance. His sandy hair, growing thin on top, was brushed carefully away from his high hollow temples. The hand he held out, with its carefully manicured nails, was stubby-fingered and shapeless. Well, well, Farley! How goes it? I've been trying to get hold of you. Want to go for a little fishing trip? He was confused because he had not spoken to Julia first. How d'ye do, Mrs. Farley? Think you could spare him for a few days? Mr. Hurst's greeting of Laurence was a combination of bluff familiarity and resentful respect. When he looked at Julia his eyes held hers in bullying admiration. Julia had never been able to say just where his elusive intimacy verged on presumption. Feeling irritated and helpless and sweetly sorry for herself, she lowered her lids. My—dear! Mrs. Hurst kissed Julia. How sweet you look! How do you do, Mr. Farley? It was nice of you to let Julia persuade you to come to us. We really feel you are showing your confidence in us. Julia, dear girl, tells me you have as much of an aversion to parties as Charles and I have. This will be a homely evening. Mr. and Mrs. Wilson are here, and there is a young Hindoo who has been giving some charming talks at the Settlement House. He speaks very poor English but he's so interested in America. He's only become
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    acquainted with afew American women. I want him to meet Julia. I think he'll amuse her too. Mrs. Hurst's short little person was draped in a black lace robe embroidered with jet. She squinted when she smiled. Minute creases appeared about her bright eyes. Her expression was gentle and deceitful. Her arms, protruding from her sleeve draperies, were thin, and their movements weak. Her wedding ring and one large diamond-encircled turquoise hung loosely on the third finger of her left hand. Her hands were meager and showed that her bones were very small and delicate. About her hollow throat she wore a black velvet band, and her cheeks, no longer firm, were, nevertheless, childishly full above it. Though she said nothing that justified it, one felt in her a sort of affectionate malice toward those with whom she spoke. In her flattering acknowledgment of Julia's appearance there was something insidiously contemptuous. Come away with me, child, and we'll dispose of that hat. Williams! She turned to the Negro servant whom Mr. Hurst had intercepted at the door. She nodded toward Mr. Farley. The Negro went forward obsequiously. Yes, Williams, take Mr. Farley's hat, Mr. Hurst said. Then, in humorous confidence, sotto voce, How about a drink, Farley? My wife has that young Hindoo here. This is likely to be a dry intellectual evening. That may suit you, but I have to resort to first aid. Want to talk to you about that fishing trip. Come on to my den with me. Shortly after this, Julia, descending the stairs with her hostess, found Laurence and Mr. Hurst in the hall again. Laurence, his lips twisted disagreeably, was listening with polite but irritating quiescence to Mr. Hurst's incessant high-pitched talk. Mr. Hurst, who had been surreptitiously glancing toward the shadowy staircase that hung above his guest's head, was quick to observe the approach of the women. He had always found fault with what he considered to be Julia's coldness, but he admired her tall figure and her fine shoulders. Hello, hello! Here they are!
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    Charles! Mrs. Hurstwas whimsically disapproving. Why haven't you taken Mr. Farley in to meet our guests? You are an erratic host. Mr. Hurst moved forward. That's all right! That's all right! Farley and I had some strategic confidences. You take him off and show him your Hindoo. I want Mrs. Farley to come out and see my rose garden, out in the court. I'm going to have a few minutes alone with her before you conduct her to the higher spheres and leave me struggling in my natural earthly environment. I won't be robbed of a little tête-à-tête with a pretty woman, just because there's an Oriental gentleman in the house who can tell her all about her astral body. Did you ever see your astral body, Mrs. Farley? Boo! Mrs. Hurst waved him off and pushed Julia toward him. Go on, if she has patience with you. But mind you only keep her there a moment. I've told Mr. Vakanda she was coming and I'm sure he's already uneasy. Rose garden, indeed! It's quite dark, Charles! Come, Mr. Farley. Put this scarf about you, dear. She took a scarf up and threw it around Julia's shoulders. Ta-ta! Mr. Hurst came confidently to Julia, and they walked out together across a glass-enclosed veranda that was brilliantly lit. Descending a few steps they were among the roses. Autumn roses, said Mr. Hurst. The bushes drooped in vague masses about them. Here and there a blossom made a pale spot among the obscure leaves. Where the glow from the veranda stretched along the paths, the grass showed like a blue mist over the earth, and clusters of foliage had a carven look. The dark wall of the next house, in which the lighted windows were like wounds, towered above them. Over it hung the black sky covered with an infinite flashing dust of stars. Julia's face was in shadow, but her hair glistened on the white nape of her neck where the black lace scarf had fallen away. Mr. Hurst had made a large sum of money from small beginnings. He would have enjoyed in peace the sense of power it gave him, and the indulgence in fine wines and foods and expensive surroundings for which he lived, but his wife prevented it. He had married her
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    when they wereboth young and impecunious. She had been a school teacher in a mid-western city. She had managed to convince him that in marrying him she conferred an honor upon him, and she succeeded now in making him feel out of place and absurd in the environment which his efforts had created, which she, however, turned to her own use. Instead of flaunting his success in boastful generosity, according to his inclination, he found himself compelled to deprecate it. He had a secret conviction that he was a man to be reckoned with, but openly, and especially before his wife's friends, he ridiculed himself, perpetrating laborious and repetitious jokes at his own expense, just as she ridiculed him when they were alone. Mrs. Hurst was chiefly interested in what she considered culture, and in welfare work, and among her acquaintances referred to her husband affectionately as if he were a child. She had no connection which would give her the entrée to socially exclusive circles, and she was wise enough not to attempt pretenses which it would have been impossible for her to sustain. Her husband's friends were mostly selfmade and newly rich. She was affable to them but maintained toward them a mild but superior reserve. She expressed tolerantly her contempt of social ostentation and suggested that among Mr. Hurst's play-fellows she was condescending from her more vital and intellectual pursuits. Men who drank and played golf or poker between the hours of business considered her brainy, but a damned nice woman. She was generous to impecunious celebrities of whom she had been told to expect success. On one occasion when she and Mr. Hurst were sailing for England she was photographed on shipboard in the company of a popular novelist. The picture of the novelist, showing Mrs. Hurst beside him in expensive furs, appeared in a woman's magazine. She had never seen the man since, but she always referred to him as a charming person. She was frequently called upon to conduct drives for charity funds. At masquerade balls organized for similar purposes her name appeared with others better known and she could honestly claim acquaintance with women whose frivolous occupations she professed to despise. She was an assiduous attendant at concerts
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    and the publiclectures which were given from time to time by men of letters or exponents of the arts. References to sex annoyed her. The vagueness of her aspirations sometimes led her into fits of depression and discouragement, but she had a small crabbed pride that prevented her from allowing any one—least of all, perhaps, her husband—to see what she felt. She was conscientiously attentive to children, but actually bored by them. She seldom thought of her own childhood, and she sentimentalized her past only when she reflected on her early girlhood and the instinctive longing for withheld refinements which had led her away from a sordid uncultured home into the profession of a teacher. Often her husband irritated her almost uncontrollably, but she never admitted that the moods he aroused in her had any significance. She was ashamed of him and called the feeling by other names. Mr. Hurst's frustrated vanity consoled itself somewhat when he was alone before his mirror, for even his wife admitted that he was distinguished looking. He consumed bottle after bottle of a prescription which, so a specialist assured him, would make his hair come back. Always gay and affectionate and generally liked, he had a secret sensitiveness that he himself was but half aware of, and which no one who knew him suspected. He had never abandoned the romantic hope that some day he would meet a woman who would understand him. It was his unacknowledged desire to have his wife's opinion of him repudiated that made him perpetually unfaithful to her. Years ago he had been astonished to discover that even the women whom his wife introduced him to, who looked down on his absence of culture, and whose intellectual earnestness really seemed to him grotesque, were quite willing to take him seriously when he made love to them. He was bewildered but elated in perceiving the vulnerability of those he was invited to revere. Once he learned this it awakened something subtle and feminine in his nature and tempted him to unpremeditated cruelties. Though his sex entanglements were, as a rule, gross and banal enough, and quickly succeeded one another, he treasured at intervals a plaintive conviction that some day he would meet the woman who had, as he
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    expressed it, theguts to love him. Musing on this, he found in it the excuse for all the unpleasing episodes in which he took part. Outwardly cynical, he was sentimental to the point of bathos. He had one fear that obsessed him, the fear of growing old, so that the woman, when she met him, might not be able to recognize him. He had always been a little afraid of Julia and had a secret desire, on the rare occasions when they met, to hurt her in some way that might force her to concede their equality. He called himself a mixture of pig and child and when he met any of his wife's high-brow friends he envied them and wanted to trick them into exhibiting something of the pig also. Julia was young and pretty. He sighed and wished her more human. He had never found her so charming as she seemed to-night. Under the accustomed stimulus of alcohol he relaxed most easily into a mood of affectionate self-pity. Without being drunk in any perceptible way, he loved himself and he loved every one, and his conviction of human pathos was strong. Julia's tense yet curiously subdued manner showed him that she was no longer oblivious to him. He fancied that there was already between them that sudden rapport which came between him and women who were sexually sensible of his personality. You aren't angry with me for taking you away like this? Julia said, How could I be? I wish all social gatherings were in the open. It seems terrible to shut one's self indoors on these beautiful nights. Charles Hurst was impelled to talk about himself. He did not know how to begin, and coughed embarrassedly. He imagined that Julia was ready to hear, and already he was grateful for the regard he anticipated. Don't mind if I light a cigar? I should like it. Don't smoke cigarettes, do you? Some of the ladies who come here shedding sweetness and light are hard smokers. Julia shook her head negatively. I don't. But you surely can't object, as a principle, to women smoking?
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    No. I thinkmy objections are chiefly—chiefly what my wife—what Catherine would call esthetic. I'm not strong on principles of any sort. Don't take myself seriously enough. Julia could make out his nonchalant angular pose as he stood looking down at her. As he held a match to his cigar the glow on his face showed his narrow regular features, his humorously ridiculing mouth, and his pale eyes caught in an unconscious expression of fright. Julia said, I'm afraid you take yourself very seriously indeed, or you wouldn't be so perpetually on the defensive. Poor Mr. Hurst! This evening she could not bear to be isolated by conventional reserves, even with him. It flattered her unhappiness to feel that he was a child. And this evening it seemed to her desperately necessary that she touch something living which would respond involuntarily to the contact. Mr. Hurst was disconcerted. He took the cigar out of his mouth and examined the glowing tip which dilated in the dark as he stared at it. Tears had all at once come to his eyes. He wondered if he were drunker than he had imagined. The moment he suspected any one of a serious interest in him it robbed him of his aplomb. Don't read me too well, Mrs. Farley. You know I'm not really much of a person. Coarse-fibered American type. No interests beyond business and all that. Good poker player. Hell of a good friend—when you let him. But commonplace. Damn commonplace. Nothing worth while at all from your point of view. They strolled along the path further into the shadows. Julia was astonished by the ill-concealed emotion in Mr. Hurst's humorous voice. His transparency momentarily assuaged the tortures of her self-distrust. How can you say that? My human predilections are not narrowed down to any particular type, I hope. Oh, well, I know—you and Catherine—miles over my head, all of it. Lectures on the Fourth Dimension. Some girl with adenoids here the other night been studying 'Einstein'. Damned if it had done her any
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    good. Yes, whatthat gal needed was somebody to hug her. Julia was conscious that he was turning toward her. Crass outlook, eh? He laughed apologetically. She probably did, Julia said. They laughed together. Mr. Hurst felt all at once unreasoningly depressed. He wanted to touch her as a child wants to touch the person who pleases it. But the sophisticated element in his nature intervened. He despised his own simplicity. Do you find yourself getting anywhere in the pursuit of the good, the true, and the beautiful? Honestly now, Mrs. Farley. I've had the whole program shoved at me—not that Catherine isn't the best of women, bless her little soul. You know the life we tired business men lead pretty much resembles that of the good old steady pack horse that does the work. We dream about green pastures and all that, but never get much closer to it. And when you get to the end of things you begin to wonder if your plodding did anybody any good—if anything ever did anybody any good. I've got no use for cynicism—consider it damn cheap. Wish some time I was a little bit more of a cynic. But I'm lost. Hopelessly lost. I take a highball every now and then because my—I think my mind hurts. He halted suddenly and they were looking into each other's vague faces. This talk getting too damn serious, eh? Something about you to-night that invites a fellow to make a fool of himself. I hope not, Julia said. I like you for talking frankly. Oh, I'm not too damn frank. We can't afford it in this world of hard knocks. Now to you, now, I'm not saying all that I'd like to, by a jugful. Then you don't make as much of a distinction between me and the crowd as I hoped. Charles had let his cigar go out. He kept turning it over and over in his stiff fingers that she could not see. He felt that only when he held a woman in his arms and she was robbed of her conventional defenses could he speak openly to her. With other attractive women he had come quickly to a point like this where he wanted to talk of
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    his inner life.He imagined it would give him relief if he could touch Julia's dress and put his head in her lap. The terrible fear of revealing himself before his wife and her friends had stimulated his imagination toward abandon. When he was a child his mother had not loved him. She was a defiant person. She was ashamed of him because he allowed himself to be victimized by all the things against which she had futilely rebelled. He had felt himself despised though he had never understood the reason. His mother found continual fault with him and never petted him. One day a girl cousin much older than he had discovered him in a corner crying and had comforted him, and had allowed him to put his head in her lap. As he had never gotten over considering himself from a child's standpoint, his adult visions always culminated in a similar moment of release. Whenever he became sentimental about a woman he imagined that he would some day put his head in her lap. He had been, in his own mind, so thoroughly convicted of weakness that the development of strength no longer appealed to him as a means of self-fulfilment. He abandoned himself to an incurable dependence for which he had not as yet found a permanent object. It eased him when he could evoke the maternal in a mistress. Aren't we all— somewhat on the defensive toward each other? he said after a minute. Julia was reminded again of what she thought to be her own tragedy. She felt reckless and wanted some one into whom to pour herself. She imagined herself lost in the dark garden, crushed between the walls and bright windows of the houses. In some indefinable way she identified herself with the million stars, flashing and remote in the black distance of the sky that showed narrowly above the roofs. Yes, she said. And so uselessly. People are so pathetic in their determination not to recognize what they are. If we ever had the courage to stop defending ourselves for a moment— But none of us have, I'm afraid. She carried the pity which she had for herself over to him. She had noticed how thin his face was, that the bold gaze with which he looked at her was only an expression of concealment, and that there were strained lines at the corners of his
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    good-tempered mouth. Yes,in the depths of his pale eyes with their conscious glint of humor there was undoubtedly something eager and almost blankly disconcerted. Charles could not answer her at once. He threw his cigar aside. His hand trembled a little. I wonder how drunk I am, he said to himself. He decided that he was helpless in the clutch of his own impulses. He thought, A damn fool now as always. Have I got this woman sized up wrong? She's a dear. Here goes. Poor little thing! Gosh, I know she can't be happy with that self-engrossed ass she's married to! In his more secret nature he was proud of his own temerity. Damn it all, Mrs. Farley—Julia— He hesitated. I've queered myself right off by calling you Julia, haven't I? His laugh was forced and unhappy. He glanced over his shoulder toward the house. Julia was alarmed by the unexpected immanence of something she was trying to ignore. She kept repeating to herself, He's a child! Her thoughts grew more disconnected each instant. She wanted to go away, yet she half knew that she was demanding of Charles the very thing that terrified her. Of course not. Mrs. Hurst calls me Julia, why shouldn't you? Her tone was intended to lift their talk to a plane of unsexed naturalness. Yes, by George, why shouldn't I! She calls you that a good deal as if she were your mother. He paused. Did you know I'd reached the ripe old age of forty-one? (He was really forty-two.) It doesn't shock me. Well, I wish it did. I don't like to be taken so damn much for granted. (He wanted to tell her that Catherine was three years older than he, but his sense of fair play withheld him.) An old man of my age has no right to go around looking for some one to understand him, has he? Why not? I'm afraid we do that to the end of time, Mr. Hurst. Say, now, honestly, Mrs. Farley—Julia—I can't lay myself wide open to anybody who insists on calling me Mr. Hurst. I feel as if I were a
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    hundred and seven.He tried to ingratiate himself with his boyishness. I haven't any objection to calling you Charles. (Julia thought uncomfortably of Mrs. Hurst and, remembering her, was embarrassed.) Don't feel hurt if I'm not able to do it at once. Certain habits of thought are very hard to get rid of. And I suppose you've been in the habit of considering me in the sexless antediluvian class! You've forgotten that Laurence—that my husband is as old as you are. When Julia mentioned her husband, Charles's impetuosity was dampened. It upset him and made him unhappy. However, he was determined to sustain his impulses. Yes, I had. Silence. Charles wanted to cry. You know I appreciate it awfully that you are willing to enter into the holy state of friendship with an obvious creature like myself. Catherine says you're a wonderful woman, and she's a damned good judge—of her own kind, that is. I'm afraid she's flattered me. I wish you weren't so humble about our friendship. I am as grateful as you are for anything genuine. Yes, I'm too confounded humble. I know I am. Always was. You know I'm not really lacking in self-respect, Miss Julia. Of course you aren't. You seem to me one of the most self- respecting people I know. Charles was silent a long time. He knew that he was being carried away on a familiar current. By God, she means it! he said to himself. He would refuse to regard anything but the present moment. How does it happen you and I never came together like this before? I'd got into the habit of thinking you were one of these icy Dianas that had an almighty contempt for any one as well rooted in Mother Earth as I am.
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    Julia laughed uncomfortably.That's a mixed metaphor. Then she said seriously, I want to understand things—not to try to escape. It seems to me we must all go back to Mother Earth if we try to do that. She added, I'm afraid we are making ourselves delinquent. We mustn't abandon Mrs. Hurst and her guests altogether. They turned toward the veranda. They were walking side by side and inadvertently Charles's hand brushed Julia's. He caught her fingers. She made a slight gesture of repulsion which he scarcely observed. Then her hand was relinquished to him. Confound these social amenities! I thought you were going to be my mother- confessor, Miss Julia. Until he touched her hand he had been conscious of their human separateness and his sensuous impulses had been in abeyance. With the feel of her flesh, she became simply the woman he wanted to kiss, the possessor of a beautiful throat, and of mysterious breasts that compelled him familiarly through the dim folds of her white dress. His acquisitive emotion was savage and childlike. Here was a strange thing which menaced and invited him. He wanted to know it, to tear it apart so that he need no longer be afraid of it. Already he annihilated it and loved it for being subject to him. He leaned toward her and when she lifted her face to him he kissed her. He felt the shudder of surprise that passed over her. Julia—don't hate me. Child, I'm going to fall in love with you! I know it! His voice was smothered in her hair. He kissed her eyes and her mouth again. Trembling, Julia was silent. He wondered recklessly if she despised him, but while he wondered he could not leave her. He felt embittered toward her because she awakened his dormant sensuality and he supposed that women like her were superior to the necessities that left him helpless. Please! Julia said. When his mouth was pressed against hers she was suffocated by the same thrill of astonishment and despair which she had experienced when she first allowed Dudley Allen to take her. When she was able to speak she said, Oh, we are so pathetic and absurd—both of us! It's so hopelessly meaningless.
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    He was excitedand elated. In a broken voice, he said, So you think I am pathetic and absurd? I am, child. I don't care! I don't care! He thought that she was referring to the general opinion of him. He hardened toward her, while, at the same moment, a wave of physical tenderness enveloped him. Stealthily, he exulted in the capacity he possessed for sexual ruthlessness. He knew she could not suspect it. He would be honest with her only when it became impossible for her to evade him. They heard footsteps and turned from each other with a common instinct of defense. Mrs. Hurst was descending the steps from the lighted porch. I have a bone to pick with that spouse of mine, she called pleasantly when she could see them. Charles had taken out a fresh cigar and was lighting a match. Hello, hello! Am I in trouble again? Charles fumbled for Julia's hand, and gave it a squeeze, but dropped it as his wife drew near. Mrs. Hurst's figure was in silhouette before them. You'll spoil my dinner party, Charles! Julia, child, I'm afraid you need reprimanding too. You have to be stern with Charles. Her tone was truly vexed, but so frankly so that it was evident she suspected nothing amiss. I'm sorry if I am in disfavor. Julia's voice was cold. In her nihilistic frame of mind she wished that her hostess had discovered the compromising situation. Julia's reply was irritating and Mrs. Hurst's displeasure inwardly deepened. She felt stirring in her a chronic distrust and animosity toward other women, but would give no credence to her own emotion. Come, child, don't be ridiculous! I suppose I can't blame Charles for trying to steal you from me. I'm sure he wanted to talk to you about himself. It's the one thing he cannot resist. She laughed, a forced pleasant little laugh, and caught Julia's arm in a determined caressing pressure. Come. We're all going to be good. Mr. Vakanda is waiting to take you in to dinner. Julia followed her toward the house. Come, Charles! Mrs. Hurst commanded him
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    abruptly over hershoulder. The manner in which she spoke to him suggested strained tolerance. Charles's immediate relief at not having been seen was succeeded by complacency. To deceive his wife was for him to experience a naïve sense of triumph. Poor little Kate! He could even be sorry for her. Julia more than ever wanted to feel that Laurence's refusal of her was forcing upon her a promiscuous and degrading attitude toward sex. She said, I'm sure the fault is mine. I couldn't resist the night and the roses. Now don't try to defend him. The roses were his excuse, not yours. Mrs. Hurst wondered how they had been able to see anything of the roses in such a light. She wished to forget about it. Mollie Wilson has been telling us how difficult the role of a mother is these days. She says she envies you May with her amenability. Lucy has some of the most startlingly advanced conceptions of what her mother should let her do. Charles, walking almost on their heels, interrupted them. It would be an insult to Ju—to Mrs. Farley if I needed an excuse for carrying her off for a minute. He cleared his throat. Say, Kate, damn it all, will you and she be upset if I call her Julia? I like her as well as you do. Again Mrs. Hurst was irritated and inexplicably disturbed. It was Charles—not Julia—of course. Any woman. He's always like that! Then I shall expect to begin calling Mr. Farley Laurence, she said acidly. She spoke confidentially to Julia. He can't resist them, dear— any of them. Pretty women. You'll have to put up with his admiration. All my nicest friends do. The dickens they do! Charles grumbled jocosely. His wife's tone made him nervous. He was suspicious of her. When they came up on the lighted veranda a maid passed them, a neat good-looking young woman in black with inquisitive eyes. Julia caught on the servant's face what seemed an expression of inquiry
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    and amusement. Charles,who had often tried to flirt with the girl, glanced at her shamefacedly and immediately lowered his gaze. Damn these women! Julia, feeling guilty and antagonistic, observed Mrs. Hurst, but found that she appeared as usual, sweet and negatively self-contained, yet suggesting faintly a hidden malice. They walked through a long over-furnished hall and entered the drawing room. The men rose: the Hindoo, good-looking but with a softness that would inevitably repel the Anglo-Saxon; Mr. Wilson, stout and jovial, his small eyes twinkling between creases of flesh, the bosom of his shirt bulging over his low-cut vest; Laurence, clumsy in gesture, kind, but almost insulting in his composure. During the evening Julia could not bring herself to meet Laurence's regard, nor did she again look directly at Mr. Hurst. Charles, after some initial moments of readjustment when he found it difficult to join in the general talk, recovered himself with peculiar ease. Indeed his later manner showed such pronounced elation that Julia wondered if it were not eliciting some unspoken comment. When he turned toward her she was aware of the furtive daring of his expression, though she refused to make any acknowledgment of it. He laughed a great deal, made boisterous jokes uttered in the falsetto voice he affected when he was inclined to comicality, and, when his jests were turned upon himself, chuckled immoderately in appreciation of his own discomfiture. The Hindoo, whose bearing displayed extraordinary breeding, had opaque eyes full of distrust. His good nature under Charles's jibes was assumed with obvious effort and did not conceal his polite contempt. During dinner and afterward Charles plied every one, and particularly the men, with drink. Mrs. Hurst had always been divided between the attractions of the elegance which demanded a fine taste in wines and liqueurs, and her moral aversion to alcohol. She never served wines when she and Charles were alone, and to-night she was provoked by his ill- bred insistence that the glasses of her guests be refilled. When the meal was over and the men had returned to the drawing room, Charles seemed to be in a state of fidgets. His face and even
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    his helpless-looking handswere flushed. He walked about continually, and was perpetually smoothing his carefully combed hair over the baldish spot on the top of his head. Mrs. Wilson, who was florid and coarsely good-looking, with her iron-gray hair, admired his distinguished figure in its well-cut clothes. His flattering manner when he talked to her made her feel self-satisfied. Julia, though she had honestly protested to Charles that she did not smoke, indulged in a cigarette. Mrs. Wilson also lit one and expelled the smoke from her pursed mouth in jerky unaccustomed puffs. Mrs. Hurst's dislike of tobacco was equal to her repugnance to alcohol. She refused to smoke but was careful to show that her distaste for cigarettes was a personal idiosyncrasy. She made little amused grimaces at the smokers and treated them as if they were irresponsible children. Mrs. Wilson, in talking to Mr. Vakanda, contrived many casual and contemptuous references to her recent experiences in Europe. She was divided between her genuine boredom with European culture and her pride in her acquaintance with it. Charles, observing Julia in this group, appreciated the distinction of her simpler, more aristocratic manner; and the clarity and frankness of her statements seemed to him to place her as a being from another world. Damn me, she's a thoroughbred! Makes me ashamed of myself, bless her soul! His emotions were too much for him. He went into his den, which was across the hall, and poured himself a drink. Fragments of the evening's conversation buzzed in his head. Julia and Mr. Wilson had disagreed as to the validity of certain phases of the newer movements in art. Mr. Wilson scoffed blatantly at all of them. Mr. Vakanda was more reserved, but one suspected that he looked upon Westerners as adolescent and treated their art accordingly. Charles, without knowing what he was talking about, had come jestingly to Julia's rescue. When he remembered how often he had joined Mr. Wilson in ribald comment on subjects which she treated as serious, he felt he had been a traitor to her. Damn my soul, I'm hard hit! I never half appreciated that girl until to-night! Don't know what the hell's been the matter with me! Overcome by his reflections, he walked to a window and stared out into the quiet
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    dimly lit street.His suddenly aroused sensual longing for Julia returned and made him embarrassed and unhappy. He set his glass down on the window ledge and passed a hand across each eye as if he were wiping something away. Damn it all, I'm in love with her all right. When the time for the Farleys' departure arrived Charles was talkative and uneasy. He clapped his hand on Laurence's shoulder. You're one of the few men who's fit to fish with, Farley. Most of 'em are too damned loud for the fish. We'll fix that little trip up yet. I suspect you of being the philosopher of this bunch anyway. I can furnish the requisite of silence, but I'm afraid it requires some peculiar psychic influence to attract fish. I haven't got it. Charles's manner was self-conscious to a degree. He spoke rapidly and unnecessarily lifted his voice. His wife watched him with a cold kind little smile of disgust. She wanted to create the impression that she understood him, but her resentment of him rose chiefly from the fact that he was incomprehensible to her. That's all right. I'll catch the fish. I'll catch the fish. Damned if I haven't enjoyed the evening. Say, Farley, Kate and I are coming over some evening and I'm going to talk to your wife. I believe she's just plain folks even if she can chant Schopenhauer and the rest of those cranks. You know I admire your brains, Miss Julia. By Jove, I do. You can give me some of the line of patter I've missed. Kate, now—Kate's got it all at her finger tips, but she's given me up long ago. Have a drink before you go, Farley? No! You know I'm a great admirer of Omar Khayyám's, Miss Julia. The rest of you high-brows seem to have put the kibosh on the old boy. He's the fellow that had some bowels of compassion in him. Knew what it was like to want a drink and be dry. Charles smoothed back his hair. His hand was trembling slightly. He looked at Julia now and then but allowed no one else to catch his eyes.
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    Laurence, holding hissilk hat stiffly in his fingers, moved determinedly toward the front door. His smile was enigmatic but his desire for escape was evident. Julia said, I'll talk to you about Schopenhauer, Mr. Hurst, and convince you that he was very far from a crank. She smiled. Yep? Well, guess I'm jealous of him. I'm willing to be taught. This business grind I'm in is converting me into pretty poor company. Not much use for a meditative mind in the stock market. Eh, Farley? The women have got it all over us when it comes to refining life. Laurence said, I imagine I know as little of the stock market as my wife, Hurst. And you must remember I'm a business woman, too. So you are. Working in that confounded laboratory. Well, I've got no excuse then. Know thyself, Charles! Mrs. Hurst shook her finger playfully. Yep. Constitutional aversion to knowing myself—knowing anything else. Looks to me as if you had picked a lemon, Kate. We must really go. Julia held out her hand. Mrs. Hurst shook hands with Julia. So delightful to have had you. I'm glad you impressed Mr. Vakanda with the significance of America in the world of art, dear. Mrs. Hurst, at that instant, disliked her guest intensely, but she preserved her smile and her delicate tactful air. Laurence shook hands with her also. His reserve appealed to her. She could be more frankly gracious with him. Charles pressed Julia's fingers lingeringly, in spite of her efforts to withdraw them. He was suddenly depressed and gazed at her with an open almost despairingly interrogative expression. Yep, damn me, Kate's right. You put the Far East in its place, Miss Julia. Did me good to see it. He giggled nervously, but his face immediately grew serious. Seeing her go away into her own strange world depleted the confidence he experienced while with her. He was oppressed by the
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