Tracing the galactic_spiral_structure_with_embedded_clusters

MNRAS 450, 4150–4160 (2015) doi:10.1093/mnras/stv840
Tracing the Galactic spiral structure with embedded clusters
D. Camargo,1,2‹
C. Bonatto1
and E. Bica1
1Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Porto Alegre 91501-970, RS, Brazil
2Colégio Militar de Porto Alegre, Ministério da Defesa – Exército Brasileiro, Av. José Bonifácio 363, Porto Alegre 90040-130, RS, Brazil
Accepted 2015 April 13. Received 2015 April 1; in original form 2015 January 16
ABSTRACT
In this work, we investigate the properties of 18 embedded clusters (ECs). The sample includes
11 previously known clusters and we report the discovery of seven ECs on WISE images, thus
complementing our recent list of 437 new clusters. The main goal is to use such clusters to
shed new light on the Galactic structure by tracing the spiral arms with cluster distances. Our
results favour a four-armed spiral pattern tracing three arms, Sagitarius–Carina, Perseus, and
the Outer arm. The Sagitarius–Carina spiral arm is probed in the borderline of the third and
fourth quadrants at a distance from the Galactic Centre of d1 ∼ 6.4 kpc adopting R = 7.2 kpc,
or d2 ∼ 7.2 kpc for R = 8.0 kpc. Most ECs in our sample are located in the Perseus arm
that is traced in the second and third quadrants and appear to be at Galactocentric distances in
the range d1 = 9–10.5 kpc or d2 = 9.8–11.3 kpc. Dolidze 25, Bochum 2, and Camargo 445
are located in the Outer arm that extends along the second and third Galactic quadrants with a
distance from the Galactic Centre in the range of d1 = 12.5–14.5 kpc or d2 = 13.5–15.5 kpc.
We find further evidence that in the Galaxy ECs are predominantly located within the thin disc
and along spiral arms. They are excellent tools for tracing these Galactic features and therefore
new searches for ECs can contribute to a better understanding of the Galactic structure. We
also report an EC aggregate located in key italicthe Perseus arm.
Key words: astronomical data bases: miscellaneous – catalogues – Galaxy: disc – open clus-
ters and associations: general – Galaxy: structure.
1 INTRODUCTION
Despite the effort that has been made to improve our understanding
of the Galactic structure, questions about the spiral arm nature
(Baba et al. 2009; Sellwood 2011; Grand, Kawata & Cropper 2012;
Mart´ınez-Garc´ıa & González-Lópezlira 2013; Roca-Fàbrega et al.
2013; Sellwood & Carlberg 2014), structure (Georgelin & Georgelin
1976; Russeil 2003; Levine, Blitz & Heiles 2006; Majaess, Turner
& Lane 2009; Hou & Han 2014), and dynamics (Fujii et al. 2011;
Binney et al. 2014) remain open. There is no consensus on the
number, pitch angle, and shape of Galactic spiral arms (Vallée 2005,
2014a,b; Hou, Han & Shi 2009; Lépine et al. 2011; Francis &
Anderson 2012; Siebert et al. 2012; Bobylev & Bajkova 2014;
Griv et al. 2014; Pettitt et al. 2014). The Sun’s location within the
dust obscured Galactic disc is a complicating factor to observe the
Galactic structure.
It is widely accepted that spiral arms are the preferred sites of star
formation and, as most stars form within embedded cluster (EC) the
arms are sites of cluster formation. Star formation may occur after
the collapse and fragmentation of giant molecular clouds (GMCs)
that occur within spiral arms transforming dense gas clumps into
E-mail: denilso.camargo@ufrgs.br
ECs. Based on the absence of massive 13
CO bright molecular clouds
in the interarm space, Roman-Duval et al. (2009) argue that molec-
ular clouds must form in spiral arms and be short-lived (less than
10 Myr). Then, the spiral arms may be traced by young star clus-
ters, especially ECs that have not had enough time to move far
from their birth places. In addition, EC parameters are derived with
good accuracy. In this sense, ECs with derived parameters can be
used to distinguish between the various theoretical models for spi-
ral arm structure (Dobbs & Pringle 2010; Sellwood 2010). Besides
constraining the spiral arm distribution with direct distances, ECs
can also contribute to kinematic modelling of the spiral structure
(Georgelin & Georgelin 1976; Russeil 2003).
We have contributed significantly to increase the number of ECs
with derived parameters (Camargo, Bonatto & Bica 2009, 2010,
2011, 2012; Camargo, Bica & Bonatto 2013, 2015a). Besides de-
riving parameters, in Camargo et al. (2015a) we discovered 437
ECs and stellar groups increasing the Galactic EC sample. Majaess
(2013) has also made a significant contribution on young clusters
(see also, Bica, Dutra & Barbuy 2003a; Bica et al. 2003b; Lada &
Lada 2003; Mercer et al. 2005; Froebrich, Scholz & Raftery 2007;
Borissova et al. 2011; Solin, Haikala & Ukkonen 2014).
The main goal of this work is deriving accurate cluster parame-
ters to use as tools to provide new constraints to better understand
the Galactic structure. We also present new ECs discovered by
C 2015 The Authors
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Galactic spiral structure 4151
Table 1. General data of the present star clusters.
Target α(2000) δ(2000) b Comments
(h m s) (◦ arcmin arcsec) (◦) (◦)
(1) (2) (3) (4) (5) (6)
IRAS 0207+6047 Cl. 2:44:37 60:59:42 136.22 1.08 also known as BDB 116 and Majaess 30
SAI 23 2:54:10 60:39:02 137.42 1.28 Glushkova et al. (2010)
Camargo 440 3:59:58 51:33:23 150.05 −1.11 This work
BDS 61 4:03:17 51:19:35 150.59 −0.94 Bica et al. (2003b)
FSR 665 4:04:59 51:31:35 150.65 −0.62 Froebrich et al. (2007)
FSR 666 4:05:51 51:28:35 150.79 −0.56 Froebrich et al. (2007)
Camargo 445 4:08:12 50:31:29 151.70 −1.02 This work (in Majaess 45)
BDS 65 4:11:10 51:09:58 151.61 −0.23 Bica et al. (2003b)
Camargo 63 6:12:01 13:39:36 196.30 −2.31 Camargo et al. (2015a)
Majaess 78 6:13:39 15:58:07 194.46 −0.87 Majaess (2013)
Dolidze 25 6:45:02 00:13:21 212.00 −1.33 Moffat & Vogt (1975)
Bochum 2 6:48:50 00:22:44 212.29 −0.40 Turbide & Moffat (1993)
NGC 2367 7:20:07 −21:52:57 235.60 −3.83 Vogt & Moffat (1972)
Notes. Cols. 2–3: central coordinates. Cols. 4–5: corresponding Galactic coordinates. Col. 6: comments.
Figure 1. WISE RGB image of the Perseus 1 aggregate: FSR 665, FSR 666, Camargo 441, Camargo 442, Camargo 443, Camargo 444, BDS 61, BDS 62, and
BDS 63.
MNRAS 450, 4150–4160 (2015)

4152 D. Camargo, C. Bonatto and E. Bica
Figure 2. WISE RGB images centred on FSR 665 (10 arcmin × 10 arcmin), Camargo 441 (7 arcmin × 7 arcmin), Camargo 442 (12 arcmin × 12 arcmin), FSR
666 (10 arcmin × 10 arcmin), Camargo 444 (10 arcmin × 10 arcmin), and Camargo 443 (10 arcmin × 10 arcmin).
ourselves as a follow up of our recent catalogue (Camargo et al.
2015a; Camargo, Bica & Bonatto 2015b). The searches for new
clusters were made by eye on WISE images.
The paper is organized as follows. In Section 2, we describe the
methods and tools employed in the cluster analyses. In Section 3,
we present the results of the cluster analysis, and derive parameters
(age, reddening, distance, core, and cluster radii). In Section 4, we
discuss the results. Finally, in Section 5 we provide the concluding
remarks.
2 METHODS OF ANALYSIS
Cluster fundamental parameters are derived using Two Micron All
Sky Survey (2MASS)1
photometry (Skrutskie et al. 2006) in the J,
H, and Ks bands, extracted in circular concentric regions centred on
the coordinates given in Table 1 and fitted with PARSEC isochrones
(Bressan et al. 2012). The fits are made by eye, taking the combined
main sequence (MS) and pre-main sequence (PMS) stellar distribu-
tions as constraints, and allowing for variations due to photometric
uncertainties and differential reddening.
The process relies on application of shifts in magnitude and colour
in the isochrone set (MS + PMS) from the zero distance modulus
and reddening until a satisfactory solution is reached. The best fits
are superimposed on decontaminated colour-magnitude diagrams
(CMDs) (Figs 3 to 9).
1 The Two Micron All Sky Survey, available at www.ipac.caltech.edu/
2mass/releases/allsky/.
To uncover the intrinsic CMD morphology, we apply a field-star
decontamination procedure. The algorithm works on a statistical
basis by measuring the relative number densities of probable cluster
and field stars in 3D CMD cells that have axes along the J, (J − H),
and (J − Ks) magnitude and colours (Bonatto & Bica 2007b; Bica,
Bonatto & Camargo 2008; Bonatto & Bica 2008, 2010). Then,
the algorithm subtracts the expected number of field stars from
each cell. It has been used in several works (e.g. Bonatto & Bica
2009, 2011a; Camargo et al. 2009, 2010, 2011, 2012, 2013; Bica &
Bonatto 2011, and references therein).
The cluster structure is analysed by means of the stellar radial
density profile (RDP) built with stars selected after applying the
respective colour–magnitude (CM) filter to the observed photom-
etry. The CM filter excludes stars with different colours of those
within the probable cluster sequences and enhances the RDP con-
trast relative to the background (e.g. Bonatto & Bica 2007a, and
references therein). Structural parameters are derived by fitting
King-like profiles to the clusters RDPs (King 1962). This proce-
dure was applied in previous works (e.g. Bonatto & Bica 2010,
2011b; Bica & Bonatto 2011; Lima et al. 2014, and references
therein).
3 RESULTS
NASA’s WISE telescope observed the whole sky in the bands W1
(3.4 µm), W2(4.6 µm), W3(12 µm), and W4 (22 µm). W1 and W2
are particularly sensitive to the EC stellar content, while W3 and
W4 show mostly dust emission.
MNRAS 450, 4150–4160 (2015)

Figure 3. 2MASS J × (J − H) and J × (J − Ks) CMDs for FSR 665.
Top panels: observed CMDs. Middle: equal area comparison field. Bottom:
field-star decontaminated CMDs. The decontaminated CMD of FSR 665
was fitted with PARSEC isochrones for both MS and PMS stars. The CM
filter used to isolate cluster stars is shown as a shaded region. We also show
the reddening vector for AV = 0–5.
Figs 1 and 2 show WISE composite images for BDS 61, BDS
65, C 441, C 442, C 443, C 444, FSR 665, and FSR 666. Figs 3–
9 show CMDs for all objects in Table 1. RDPs are presented in
Figs 10 and 11. Fundamental parameters are shown in Table 2 and
the structure for clusters that can be described by a King law are
shown in Table 3.
3.1 Previously known star clusters
IRAS 0207+6047 Cluster (BDB 116): we derived an age of
2 ± 1 Myr for a distance of d = 2.8 ± 0.4 kpc (Fig. 9). The
structure of this object points to a cluster, but cannot be fitted by a
King-like function (Fig. 11).
SAI 23: fitting isochrones to cluster decontaminated CMD
(Fig. 6), we derive an age of ∼4 Myr for a distance of
d = 2.8 ± 0.4 kpc. From the RDP (Fig. 10), we found
Rc = 0.85 ± 0.1 pc, σ0K = 25.2 ± 3.6 stars pc−2
, and a cluster
RDP radius of 10.01 ± 2.3 pc.
BDS 61: is an EC located in the Perseus arm presenting an O-
type star in the central region (R < 1 arcmin), as shown by the
decontaminated CMD (Fig. 8). We derive an age of ∼1 Myr for a
Figure 4. Same as Fig. 3 for Camargo 442.
d = 2.7 ± 0.3 kpc. Despite a dip in the inner RDP region, the struc-
ture of BDS 61 is consistent with an EC in the early evolutionary
phase (Fig. 11).
FSR 665: located close to the Perseus arm, this object is a promi-
nent EC with a relatively well-developed MS and a populous PMS
(Fig. 3). The best fit of PARSEC isochrones (MS + PMS) suggest
an age of ∼4 Myr and a d = 2.9 ± 0.3 kpc. The presence of other
ECs creates a bump in the FSR 665 RDP (Fig. 10).
FSR 666: WISE images (Figs 1 and 2) of FSR 666 point to
ongoing star formation. The decontaminated CMD and the respec-
tive MS and PMS isochrone fits provide an age of ∼1 Myr for a
d = 2.8 ± 0.7 kpc, which put this EC in the Perseus arm. The
RDP of FSR 666 is irregular and cannot be fitted by a King-like
profile (Fig. 11), but suggests a cluster. Note that young clusters
lack the span of time to be an isothermal sphere.
BDS 65: located in the Perseus arm at a d = 2.5 ± 0.4 kpc.
The object is a prominent EC, as indicated by CMDs (Fig. 8) and
RDP (Fig. 11). The decontaminated CMD of BDS 65 shows a
very reddened O star within the central region (R < 1 arcmin)
and suggests an age of ∼1 Myr. The RDP presents a high con-
trast with respect to the background, but King profile cannot be
fitted.
Camargo 63: recently discovered by Camargo et al. (2015a) this
EC is confirmed as a cluster in this work. The decontaminated CMD
of C 63 fitted by PARSEC isochrones (Fig. 6) suggests an age of
MNRAS 450, 4150–4160 (2015)

Figure 5. Same as Fig. 3 for NGC 2367. The squares on the decontaminated
CMDs indicate B stars.
∼3 Myr and present three B stars in the central region. We found a
distance from the Sun of 3.3 ± 0.5 kpc, close to the Perseus arm.
The RDP of C 63 is irregular (Fig. 11), certainly owing to absorption
effects in the 2MASS bands.
Majaess 78: our analyses (Fig. 9) suggest that this EC lies in the
Perseus arm at a distance of 3.2 ± 0.5 kpc and presents an age of
∼3 Myr. Fig. 11 shows the irregular RDP for Majaess 78.
Dol 25: Moffat & Vogt (1975) derived a distance from the Sun of
5.25 kpc and Lennon et al. (1990) a d = 5.5 ± 0.5 kpc. We argue
that Dol 25 is related to Sh2-284 and lies at d = 5.5 ± 0.8 kpc. We
obtained an age of ∼2 Myr for Dol 25. Five B-type stars are found
in the central region of the cluster. The inner region of the cluster
(R < 1 arcmin) contains four of the massive B stars shown in the de-
contaminated CMD (Fig. 8). The RDP fitted by a King-like profile
(Fig. 10) gives Rc = 0.31 ± 0.06 pc, σ0K = 21.42 ± 5.98 stars pc−2
,
and RRDP = 8.05 ± 3.22 pc. The RDP irregularities at large radii
are probably caused by stellar overdensities in the Dol 25 neigh-
bourhood, probably related to sequential star formation.
Bochum 2: Stephenson & Sanduleak (1971) found a distance
from the Sun of 5.5 kpc for Bo 2. subsequently, Moffat, Jackson &
Fitzgerald (1979) estimated a distance of 4.8 kpc. Munari & Carraro
(1995) estimated an age of 7 Myr for a distance from Sun of ∼6 kpc.
Our analyses of the decontaminated CMD of Bo 2 (Fig. 8) suggest
an age of ∼5 Myr and a d = 7.9 ± 1.1 kpc. This EC lies in
Figure 6. 2MASS CMDs of the ECs SAI 23, Camargo 63, and Camargo
440. Top panels: observed CMDs J × (J − H). Middle panels: equal area
comparison field. Bottom panels: field-star decontaminated CMDs fitted
with MS and PMS PARSEC isochrones. The CM filters used to isolate cluster
stars are shown as shaded regions (only for clusters with RDP following a
King-like profile). The squares on the decontaminated CMDs indicate B
stars. We also present the reddening vector for AV = 0 to 5.
the Outer arm. The O stars shown in the decontaminated CMD are
members of binary systems and are located in the inner cluster radius
(R < 1 arcmin). The RDP of Bo 2 provides Rc = 0.57 ± 0.16 pc,
σ0K = 7 ± 3 stars pc−2
for a cluster radius of 9.2 ± 2.3 pc.
NGC 2367: Carraro et al. (2005) derived an age of 5 Myr and a
distance from the Sun of 1.4 kpc for NGC 2367 while Santos-Silva &
Gregorio-Hetem (2012) derived an age of 3 Myr and d ∼ 2.2 kpc.
We estimated an age of 2 ± 1 Myr and d = 3.4 ± 0.7 kpc for
NGC 2367. It presents five B stars in the central region, two of them
with J excess (Fig. 5). The cluster shows a smooth RDP (Fig. 10)
with σ0K = 24.64 ± 13.64 pc−2
and RRDP = 4.75 ± 0.47 pc.
Both, CMDs and RDP suggest that this EC will evolve to become
a classical OC.
3.2 Present work discoveries
We discovered in this work seven new ECs. The newly found
star clusters complement our previous catalogue (Camargo et al.
2015a,b).
Camargo 440: this new EC is located in the Perseus arm at a
d = 2.6 ± 0.4 kpc. Based on the cluster CMD (Fig. 6), we
MNRAS 450, 4150–4160 (2015)

Figure 7. Same as Fig. 6 for Camargo 444, Camargo 445, and Camargo
441.
estimated an age of ∼1 Myr. The overdensity in the outermost RDP
of C 440 is due to King 7 (Fig. 11).
Camargo 441: this EC present an age of 1 Myr and is located at
a d = 2.5 ± 0.5 kpc in the Perseus arm. Both the decontaminated
CMD (Fig. 8) and RDP (Fig. 11) suggest a small poor cluster.
Camargo 442: this EC lies in the Perseus arm at a
d = 2.7 ± 0.4 kpc. The best isochrone fit (Fig. 4) provides an
age of ∼1 Myr. The irregular RDP (Fig. 11) does not follow a
cluster-like profile.
Camargo 443: this previously unknown EC lies in the Perseus
arm at a distance of d = 3.1 ± 0.7 kpc. C 443 is ∼3 Myr old
and presents a well-defined MS and PMS with very reddened stars
(Fig. 9). Its RDP suggests a cluster, but does not follow a King-like
profile (Fig. 11), which is expected for young clusters (Camargo
et al. 2015a).
Camargo 444: lies at a distance of d = 3.0 ± 0.7 kpc in the
Perseus arm. The analysis of the images (Figs 1 and 2) and de-
contaminated CMD (Fig. 7) suggests ages of ∼1–2 Myr. The RDP
(Fig. 10) points to cluster, but cannot be fitted by King-like profile.
Camargo 445: we derive via the decontaminated CMD (Fig. 7)
analysis an age of ∼1 Myr and a d = 6.3 ± 0.9 kpc, which sets
this EC in the Outer arm. The RDP of C 445 is irregular and cannot
be fitted by a King-like profile (Fig. 11).
Camargo 446: this object is an EC with an age of ∼1 Myr and
is located in the Perseus arm at a distance of d = 2.9 ± 0.4 kpc
(Fig. 9). The RDP does not follow a King law (Fig. 11).
Figure 8. Same as Fig. 6 for BDS 61, BDS 65, Dolidze 25, and Bochum 2.
The circles on the decontaminated CMDs indicate O stars and the squares
B stars.
4 DISCUSSION
The present analysis places the ECs BDS 61, C 441, C 442, C
443, C 444, FSR 665, and FSR 666 in the same region in the
Perseus arm. Fig. 1 presents this star cluster aggregate (hereafter
Perseus 1 cluster aggregate) formed by ECs with similar ages. Such
groups were predicted or observed by Efremov (1995), de la Fuente
Marcos & de la Fuente Marcos (2008, 2009a, 2010), Guillout et al.
(1998), Fellhauer & Kroupa (2005), Camargo et al. (2011, 2012,
2013, 2015a) and Feigelson et al. (2011). Several large dust emis-
sion bubbles in WISE images connect these clusters, suggesting
second generation effects (Camargo et al. 2015a). This leaves open
the possibility that an entire GMC may fragment almost simulta-
neously forming a large EC aggregate. The spiral arms may play
an important role in the erosion of a GMC triggering massive star
formation in the whole cloud. Camargo et al. (2012) pointed out
the star formation within cluster groups. They suggest that an ir-
regular GMC (or complex) may form massive stars simultaneously
and their winds and/or supernova explosions may produce a second
generation of massive stars propagating the star formation and pro-
ducing star clusters with similar age. The ECs formation is quite
rapid and probably range from 0.5–5 Myr (Lada & Lada 2003;
Allen et al. 2007; Tamburro et al. 2009; Santos-Silva & Gregorio-
Hetem 2012; Camargo et al. 2013). According to Camargo et al.
(2013), the deep embedded phase for most Galactic ECs ends be-
fore 3 Myr (Ballesteros-Paredes & Hartmann 2007; de la Fuente
MNRAS 450, 4150–4160 (2015)

Figure 9. 2MASS CMDs for the clusters Camargo 446, Majaess 78, IRAS
0207 + 6047 Cl., FSR 666, and Camargo 443. Top panels: observed CMDs
J × (J − Ks). Middle panels: equal area comparison field. Bottom pan-
els: field-star decontaminated CMDs fitted with MS and PMS PARSEC
isochrones. We also present the reddening vector for AV = 0–5.
Marcos & de la Fuente Marcos 2009b). Bally (2008) argue that OB
associations consist of sub-groups, clusters, and sub-clusters with
no preferred scale (Jose et al. 2013). The GMC W51 with an EC
aggregate distributed inside ≈100 pc may be an example of this
scenario (Kumar, Kamath & Davis 2004). Several H II regions in
Anderson et al. (2014) are within complexes. Several EC candidates
from Solin et al. (2014) have neighbours. The present Perseus 1 ag-
gregate of ECs is similar to the Auriga 1 aggregate, recently found
by Camargo et al. (2012).
The large-scale cluster formation within the structure shown in
Fig. 1 may be a consequence of the Perseus arm gas compression,
but the small-scale cluster distribution suggest sequential formation.
Dol 25 is located in the centre of a large bubble and apparently
is triggering a sequential star formation event, forming a second
EC generation (Lee, Murray & Rahman 2012; Camargo, Bica &
Bonatto 2015a). In this sense, EC aggregates appear to be formed
in sequential events.
BDS 61, BDS 65, Dol 25, and Bochum 2 present massive stars
located within their central region (R < 1 arcmin).
Figure 10. RDPs for the ECs NGC 2367, FSR 665, SAI 23, Bochum
2, Dolidze 25, and Camargo 444. Brown horizontal shaded region: stellar
background level measured in the comparison field. Grey regions: 1σ King
fit uncertainty.
4.1 Galactic distribution
Fig. 12 updates the previous version in fig. 14 in Camargo et al.
(2013). Black circles are clusters in this work and brown circles
ECs from our previous studies (Camargo et al. 2010, 2011, 2012,
2013, 2015a).
Following Camargo et al. (2013), most ECs are found in the thin
disc within 250 pc from the Galactic mean plane (Vallenari, Bertelli
& Schmidtobreick 2000; Siebert, Bienaymé & Soubiran 2003) as
shown in panels (b), (c), and (d). The ZGC distribution of the present
cluster sample (Table 2) is consistent with Buckner & Froebrich
(2014) result for the Galactic cluster scaleheight. However, some
ECs are found at large distances from the Galactic plane (Fig. 12),
mainly in the spiral arms (Camargo et al. 2013).
The Sagittarius–Carina spiral arm is well traced by the sub-
sample of clusters with derived parameters in our recent discovered
ECs (Camargo et al. 2015a). This spiral arm is presently probed
near the borderline of the third and fourth quadrants at a distance
from the Galactic Centre of d1 ∼ 6.4 kpc or d2 ∼ 7.2 kpc.
Most ECs in the present sample are distributed in the second and
third quadrants along the Perseus arm. In this region the Perseus
arm is located at Galactocentric distances in the range of 9 kpc in
the second quadrant to 10.5 kpc in the third quadrant for a distance
MNRAS 450, 4150–4160 (2015)

Figure 11. RDPs for the confirmed clusters BDS 65, Camargo 443, Ca-
margo 445, BDS 61, IRAS 0207+6047 Cl., Camargo 441, Camargo 63,
Majaess 78, FSR 666, Camargo 440, Camargo 446, and Camargo 442.
of the Sun to the Galactic Centre of 7.2 kpc or in the range of
9.8–11.3 kpc for R = 8.0 kpc.
The presence of molecular clouds beyond the Perseus arm is
known since a few decades (Digel, de Geus & Thaddeus 1994;
Heyer & Terebey 1998). Kaltcheva & Hilditch (2000) proposed
the existence of a spiral arm in the Outer Galaxy, the Outer arm.
Since then, several works have been developed, but the Outer arm is
still not fully traced (Russeil 2003; Pandey, Sharma & Ogura 2006;
Honma et al. 2007; Russeil, Adami & Georgelin 2007; Hachisuka
et al. 2009, and references therein). In Camargo et al. (2013), based
on the distribution of ECs, we confirmed that the Outer arm extends
along the second and third Galactic quadrants with Galactocen-
tric distances in the range of 12.5–14.5 kpc for R = 7.2 kpc or
13.5–15.5 kpc for R = 8.0 kpc. The distance from the Sun and
Galactocentric radii derived for the Outer arm in this work agrees
with Hachisuka et al. (2009) results. There is a large discrepancy
between the stellar Outer arm and the gaseous Outer arm with dis-
tance larger than 20 kpc, but it appears to be a common feature for
large spiral galaxies (McClure-Griffiths et al. 2004).
5 CONCLUDING REMARKS
In this work, we investigate the properties of 18 ECs. Besides 11
previously known clusters we discovered seven ECs, some of them
Table2.DerivedfundamentalparametersforECsinthepresentstudy.
ClusterE(J−H)(mag)Age(Myr)d(kpc)RGC(kpc)xGC(kpc)yGC(kpc)zGC(pc)RGC(kpc)xGC(kpc)yGC(kpc)zGC(pc)
(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)
IRAS0207+6047Cl.0.28±0.022±12.8±0.49.4±0.3−9.25±0.281.94±0.2752.8±7.410.51±0.28−10.33±0.281.94±0.2752.89±7.42
SAI230.28±0.024±12.7±0.49.4±0.3−9.19±0.281.81±0.2559.8±8.410.43±0.28−10.27±0.281.81±0.2559.86±8.40
Camargo4400.27±0.021±0.52.6±0.49.5±0.3−9.46±0.311.29±0.18−50.2±7.010.62±0.31−10.54±0.311.29±0.18−50.20±7.04
BDS610.23±0.032±12.7±0.39.7±0.3−9.59±0.3351.34±0.18−44.7±6.210.76±0.33−10.68±0.331.32±0.18−21.27±2.95
Camargo4410.21±0.031±0.52.5±0.59.5±0.4−9.44±0.421.26±0.24−36.0±6.810.59±0.41−10.52±0.421.26±0.24−36.01±6.76
Camargo4420.32±0.021±12.7±0.49.6±0.3−9.55±0.341.31±0.19−37.3±5.510.71±0.34−10.63±0.341.31±0.19−37.27±5.47
FSR6650.32±0.034±1.52.9±0.39.9±0.2−9.77±0.21.43±0.14−31.7±3.010.95±0.24−10.85±0.241.43±0.14−31.67±3.02
FSR6660.27±0.031±12.8±0.79.8±0.6−9.70±0.581.39±0.32−27.9±6.510.87±0.57−10.78±0.581.39±0.32−27.77±6.47
Camargo4430.30±0.023±13.1±0.710.1±0.6−9.97±0.651.52±0.36−35.1±8.311.16±0.64−11.05±0.651.52±0.36−35.10±8.26
Camargo4440.29±0.032±13.0±0.710.0±0.6−9.88±0.631.47±0.34−27.6±6.510.93±0.35−10.84±0.361.40±0.20−26.34±3.69
Camargo4450.26±0.021±0.56.3±0.913.1±0.7−12.76±0.782.98±0.42−112.1±15.714.16±0.76−13.84±0.782.98±0.42−112.05±15.71
BDS650.22±0.031±12.5±0.49.5±0.3−9.43±0.331.20±0.18−10.1±6.210.58±1.02−10.51±1.021.20±0.55−10.15±4.68
Camargo4460.14±0.011±0.52.9±0.410.1±0.4−10.05±0.39−0.68±0.09−59.4±8.211.15±0.39−11.13±0.39−0.68±0.09−59.44±8.24
Camargo630.14±0.023±13.3±0.510.5±0.4−10.43±0.45−0.94±0.13−131.2±18.411.54±0.45−11.51±0.45−0.94±0.13−131.22±18.40
Majaess780.22±0.025±33.2±0.510.3±0.5−10.29±0.50−0.79±0.13−47.5±7.711.39±0.50−11.37±0.50−0.79±0.13−47.54±7.74
Dolidze250.25±0.012±15.5±0.812.3±0.7−11.93±0.66−2.94±0.66−128.9±18.113.34±0.65−13.01±0.66−2.94±0.41−128.85±18.07
Bochum20.30±0.015±17.9±1.114.5±0.9−13.89±0.92−4.22±0.58−55.1±7.615.56±0.90−14.97±0.92−4.22±0.58−55.11±7.64
NGC23670.12±0.012±13.3±0.79.5±0.4−9.06±0.42−2.69±0.62−218.6±50.410.50±0.44−10.14±0.42−2.69±0.62−218.60±50.35
Notes.Col.2:E(B−V)inthecluster’scentralregion.Col.3:age,from2MASSphotometry.Col.4:distancefromtheSun.Col.5:RGCcalculatedusingR=7.2kpcforthedistanceoftheSuntotheGalactic
Centre(Bicaetal.2006).Cols.6–8:GalactocentriccomponentsusingR=7.2kpc.Col.9:RGCcalculatedusingR=8.3kpcforthedistanceoftheSuntotheGalacticCentre.Cols.10–12:Galactocentric
componentsusingR=8.3kpc.
MNRAS 450, 4150–4160 (2015)

Table 3. Structural parameters for clusters in the current sample.
Cluster (1 arcmin) σ0K Rcore RRDP σ0K Rcore RRDP
(pc) (∗ pc−2) (pc) (pc) (∗ arcmin−2) (arcmin) (arcmin)
(1) (2) (3) (4) (5) (6) (7) (8)
SAI 23 0.77 25.23 ± 3.63 0.85 ± 0.10 10.01 ± 2.31 14.96 ± 2.15 1.11 ± 0.13 13.0 ± 3.0
FSR 665 0.85 25.92 ± 8.4 0.67 ± 0.16 6.97 ± 2.55 18.73 ± 6.07 0.79 ± 0.19 8.2 ± 3.0
Dol 25 1.61 21.42 ± 5.98 0.31 ± 0.06 8.05 ± 3.22 55.52 ± 15.50 0.19 ± 0.04 5.0 ± 2.0
Bo 2 2.29 7.03 ± 2.97 0.57 ± 0.16 9.16 ± 2.30 36.85 ± 15.59 0.25 ± 0.07 4.0 ± 1.0
NGC 2367 0.95 24.64 ± 13.64 0.31 ± 0.12 4.75 ± 0.47 22.24 ± 12.31 0.33 ± 0.13 5.0 ± 0.5
Figure 12. Spatial distribution of the ECs in schematic projection of the Galaxy as seen from the north pole, with 7.2 kpc as the Sun’s distance to the Galactic
Centre. Black circles are ECs in this work, squares are ECs from Camargo et al. (2015a), and brown circles are ECs in our previous works.
MNRAS 450, 4150–4160 (2015)

forming a prominent EC aggregate located in the Perseus arm. The
present results indicate that in the Galaxy ECs are predominantly
located in the spiral arms. Thus, new searches for ECs are important,
since they may contribute to the further understanding of the Galaxy
structure.
The ECs in the present sample are distributed along three arms,
Sagitarius-Carina, Perseus, and Outer arm.
The Sagittarius–Carina spiral arm in the region traced by our EC
sample is at a Galactocentric distance of ∼6.4 kpc for R = 7.2 kpc
or ∼7.2 kpc for R = 8.0 kpc.
The Perseus arm along the second and third quadrants presents a
Galactocentric distance in the range 9–10.5 kpc for a distance of the
Sun to the Galactic Centre of 7.2 kpc or in the range of 9.8–11.3 kpc
for a R = 8.0 kpc.
The parameters derived for Bochum 2 and C 445 reinforce our
previous results for the Outer arm. This feature extends along the
second and third Galactic quadrants with a distance from the Galac-
tic Centre in the range of 12.5–14.5 kpc for R = 7.2 kpc or
13.5–15.5 kpc for R = 8.0 kpc.
We find that in the Galaxy most ECs are distributed within the
thin disc (∼250 pc) and along spiral arms. However, there occur
ECs with zGC > 500 pc.
Most massive stars identified in the present EC sample are located
within the central region of the cluster (R < 1 arcmin).
ACKNOWLEDGEMENTS
We thank an anonymous referee for important comments and sug-
gestions. This publication makes use of data products from the
2MASS, which is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Centre/California Insti-
tute of Technology, funded by the National Aeronautics and Space
Administration and the National Science Foundation. CB and EB
acknowledge support from CNPq (Brazil).
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