Issue |
A&A
Volume 498, Number 1, April IV 2009
|
|
---|---|---|
Page(s) | 109 - 114 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200911945 | |
Published online | 11 March 2009 |
A third red supergiant rich cluster in the Scutum-Crux arm
J. S. Clark1 - I. Negueruela2 - B. Davies3,4 - V. M. Larionov5,6 - B. W. Ritchie1,7 - D. F. Figer4 - M. Messineo4 - P. A. Crowther8 - A. A. Arkharov9
1 - Department of Physics and Astronomy, The Open
University, Walton Hall, Milton Keynes MK7 6AA, UK
2 -
Departamento. de Física, Ingeniería de Sistemas y
Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080
Alicante, Spain
3 -
School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
4 -
Chester F. Carlson Centre for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY 14623, USA
5 -
Astronomical Institute of St. Petersburg University, Petrodvorets, Universitetsky pr. 28, 198504 St. Petersburg, Russia
6 -
Isaac Newton Institute of Chile, St. Petersburg branch, Russia
7 -
IBM United Kingdom Laboratories, Hursley Park, Winchester, Hampshire SO21 2JN, UK
8 -
Department of Physics & Astronomy, University of Sheffield, Sheffield S3 7RH, UK
9 -
Pulkovo Astronomical Observatory, 196140 St. Petersburg, Russia
Received 25 February 2009 / Accepted 8 March 2009
Abstract
Aims. We aim to characterise the properties of a third massive, red supergiant dominated galactic cluster.
Methods. To accomplish this we utilised a combination of near/mid- IR photometry and spectroscopy to identify and classify the properties of cluster members, and statistical arguments to determine the mass of the cluster.
Results. We found a total of 16 strong candidates for cluster membership, for which formal classification of a subset yields spectral types from K3-M4 Ia and luminosities between
-4.8 for an adopted distance of
kpc. For an age in the range of 16-20 Myr, the implied mass is 2-
,
making it one of the most massive young clusters in the Galaxy. This discovery supports the hypothesis that a significant burst of star formation occurred at the base of Scutum-Crux arm between 10-20 Myr ago, yielding a stellar complex comprising at least
of stars (noting that since the cluster identification criteria rely on the presence of RSGs, we suspect that the true stellar yield will be significantly higher). We highlight the apparent absence of X-ray binaries within the star formation complex and finally, given the physical association of at least two pulsars with this region, discuss the implications of this finding for stellar evolution and the production and properties of neutron stars.
Key words: stars: supergiants - Galaxy: open clusters and associations: individual: ScutumCrux arm - stars: late-type
1 Introduction
The vigorous star formation that characterises starburst galaxies
results in the production of extended complexes of young massive
stellar clusters, which
span hundreds of parsecs but appear to have formed over a limited
time-frame (20 Myr; Bastian et al. 2005). With masses
>
,
analogues of such constituent clusters had been
thought to be absent from our own
Galaxy. However near-IR observations revealed that the Galactic Centre
hosts 3 such young massive
clusters (Figer et al. 1999, 2002, 2004),
while detailed study of Westerlund 1 suggests a mass of the
order of
(Clark et al. 2005). Such discoveries raise the exciting
prospect of directly determining such
fundamental properties as their (Initial) Mass Function; currently
impossible for unresolved extragalactic examples.
Furthermore, their presence in the Galaxy
permits the detailed investigation of massive stellar evolution, since
their high mass
yields significant numbers of rare spectral types in a co-eval
setting of uniform metallicity. With ages
<Myr, Westerlund 1 and the Galactic Centre clusters
provide valuable insights into the properties and
evolutionary pathways of massive (>
)
stars. Recently,
studies by Figer et al. (2006, F06) and Davies et al. (2007,
2008; D07 and D08 respectively) have revealed two
further massive clusters dominated by red supergiants (RSGs) at the
base of Scutum-Crux arm - RSGC1 (
Myr;
)
and RSGC2
(
Myr;
). Collectively, both clusters
sample a somewhat lower range of stellar masses, hosting 40 RSGs with
;
of particular interest since such stars are thought to be type II SNe
progenitors (Smartt et al. 2008).
In this paper we report the discovery of a third massive, RSG
dominated cluster, RSGC3, also located at the base of the Scutum-Crux arm.
Identified visually in GLIMPSE/Spitzer mid-IR images (Benjamin et al.
2003) as a concentration
of bright stellar sources at
,
we utilised near-IR photometry to identify potential cluster
members, a subset of which were subsequently observed
spectroscopically to
provide a firm classification. Finally, a synthesis of these data
were used to constrain the bulk properties of the cluster and
individual stars
within it, enabling a comparison to RSGC1 and 2 and a
characterisation of the star forming environment they delineate.
2 The RSG candidate sample
![]() |
Figure 1:
Near-IR |
Open with DEXTER |
As can be seen from the near IR images of RSGC1-3 (F06, D07, Figs. 1 and 2) it is extremely difficult to determine a physical extent for such (putative) clusters since, with the exception of the RSGs, no other cluster population is readily visible as an overdensity with respect to the stellar field population. If kinematic information is available, it is possible to identify a co-moving, physical association of RSGs, in order to discriminate between cluster and field stars (e.g. D07). However, the spectroscopic data presented here are of insufficient resolution to extract the radial velocity of cluster members, while, unlike RSGC1 and 2 (F06; D07), we find no maser emission from any cluster members which would also provide kinematic constraints (Verheyen et al. in prep.); thus we are forced to utilise photometric data to construct a list of candidate cluster members.
Based on the spatial concentration of bright red stars, we start by
taking 2MASS photometry for stars within
of the position
of Star 1 (RA: 18
45
23.60
,
Dec:
), selecting only
stars with quality flags ``AAA'' and error
.
The
ten bright stars defining the spatial concentration form a
well-separated group
in the
diagram (Fig. 3), around
.
This grouping is also present in the
diagram, centred around
.
We then calculate the reddening-free parameter
.
Early-type stars have
,
while most bright field stars have
,
corresponding to red giants (Indebetouw et al. 2005;
Negueruela & Schurch 2007). All ten stars form a
clearly separated grouping in this diagram, with values 0.2-0.4,
typical of supergiants. There is one more star in this clump,
S14, which has redder
and
.
The
only other star of comparable
in the field, S28, has
,
typical of an early-type star.
Considering the large number of bright stars in the field and the
spatial extent of other starburst clusters in the area (e.g. F06,
D07), we extended
the search to
.
The group in the
and
diagrams,
which we consider to comprise prime cluster members now includes S11,
S12, S13 and S15 (Table 1).
![]() |
Figure 2:
Finding chart for RSGC3, with the stars listed in Table 1
indicated. The finder comprises a |
Open with DEXTER |
![]() |
Figure 3:
Colour magnitude plot for stars within
|
Open with DEXTER |
Table 1:
Summary of RSG candidates and their properties.
Top panel:
the core group of 15 photometrically selected stars
regarded as prime cluster candidates; second panel: likely cluster
members; third panel: stars of interest, as defined in Sect. 2 and fourth panel: likely foreground
RSG identified by spectroscopy and included for completeness.
Note that based on spectroscopy, we also consider it likely that star S16 is a cluster member and treat it as such in the text. Co-ordinates
and near-IR magnitudes are from 2MASS, with mid-IR (5-25
m)
magnitudes from the Galactic plane surveys of GLIMPSE/ Spitzer
(Benjamin et al. 2003) and the Midcourse Source
Experiment (MSX) (Egan et al. 2001). We also
provide the dereddened MSX (A-C) colour (adopting the
prescription of Messineo et al. 2005), which is
a diagnostic of emission from circumstellar dust (and hence proxy for
mass loss) and the spectral type of the stars, where available (Sect. 3).
A number of objects - S14 (found in the cluster core), S15-16 and S18-22 -
have
similar to the above, but with redder -
- colours; the separation between these
stars and the main plume of red giant stars in terms of
is not as well defined as that for the prime cluster candidates.
We identify these as likely cluster members, with the difference in
colours potentially due
to excess reddening with respect to the core members (noting that
significant differential reddening is also observed for RSGC2; D07).
Finally, for completeness, with the inclusion of S23 and S27, these
stars form a well defined group in the
diagram. However, only S14, S15, S16 and S18 are grouped in
the
,
while S19, S20, S22, S24, S25,
S26 and S27 form a second, distinct
group in
.
Therefore, given their magnitudes and red
colours, we identify S23-27 as
potential objects of interest, but as with our second group, they require
spectroscopic follow up to ascertain their nature and relationship to
RSGC3 (Table 1).
To summarise, based solely on their near-IR properties we identify a
core group of 15 prime candidate
cluster members, a second group of 7 likely cluster members, and a
final group of 5 bright red stars
that deserve investigation within
of the nominal
cluster core (Table 1).
3 Spectroscopic results and analysis
Based on their photometric properties, initial low resolution ()
observations of 17 stars were made with the
IR imaging camera SWIRCAM+HK grism, mounted on the AZT-24 1.1 m telescope at Campo Imperatore on 2006 September 3 and 4.
Subsequently, higher resolution observations of 8 targets, made in
the flexible observing mode, were obtained with the
m imaging spectrometer UIST,
mounted on the United
Kingdom Infra-Red Telescope on 2007 June 8 and 21 (Program ID
U/07A/15). The Long K grism was used with the 4-pixel slit, giving wavelength coverage from 2.20-
m with a
resolution
.
Data reduction was accomplished via the methodology described in Clark et al. (2003), and the spectra are presented
in Fig. 2.
Of the 17 low resolution spectra, 16 show deep CO bandhead absorption, characteristic of late type stars (Fig. 4). Of these, 14 are photometrically defined core cluster members; one, S16, a likely member, and the final star, S28, appears to be a foreground object based on its near-IR colours (Table 1). Following the methodology of F06 and D07, it is possible to use the strength of the CO bandheads to provide a spectral and luminosity classification for the stars. However this requires a robust determination of the stellar continuum, which proved impossible for the low resolution spectra, and consequently was only attempted for the subset of 8 stars for which medium resolution data were available.
We find all 8 stars - S2-5 and S7-10 - to be supergiants, with spectral types ranging from K5-M4 Ia; the resultant temperatures (and associated errors) are summarised in Table 2. Given the equivalence of the low resolution spectra of these stars with those of S1, S6, S11 and S13-16, we conclude that these stars are likewise RSGs. Thus these results provide strong support for the identification of S1-15 as bona fide cluster members based on both spectroscopic and photometric criteria, with S16 possibly a more heavily reddened cluster member. For the remainder of the paper we therefore count these sixteen stars as cluster members; in a future paper we will use high resolution spectroscopy to confirm such a physical association (Davies et al. in prep.).
![]() |
Figure 4: Montage of low (Campo Imperatore; black) and medium resolution (UKIRT; red) spectra of selected photometric targets, revealing the prominent CO bandhead absorption. |
Open with DEXTER |
Table 2: Summary of the stellar properties of the 8 RSGs for which accurate spectral classification was possible, assuming a cluster distance of 6 kpc.
The current lack of kinematic data precludes the determination of the
cluster distance via comparison to the Galactic rotation curve, and hence the
luminosity, age and initial mass of the cluster members (since RSGs
span a wide range of luminosities
(
-5.8; Meynet & Maeder
2000). Nevertheless, we find a mean value of
,
from which we may infer
and hence, assuming a canonical
1.8 mag. extinction per kpc,
an upper limit to the distance to RSGC3 of
7.2 kpc (Rieke & Lebofsky 1985; Egan et al.
2002). Such a value is
entirely consistent with a location of RSGC3 at a similar distance to
RSGC1 and 2 at the end of the Galactic Bar (
kpc and 5.83
1.91-0.76 kpc respectively,
D08); we thus adopt a distance of
kpc for the remainder of this work.
At such a distance, utilising the temperature/spectral type calibration and resultant bolometric corrections
of Levesque et al. (2005), we find
-4.8 for
S2-5 and 7-10 and corresponding ages and initial masses of
Myr and
10-
(Fig. 5).
Finally, the observed range of the dereddened [8]-[12] colour index -
MSX
-1.44 (Table 1) - for the 7 (candidate)
cluster members for which it may be determined is directly comparable
to that found by D07 for RSGC2.
Sampling the broad silicate emission feature, this provides a measure
of the mass loss rate via the dust content of the circumstellar
environment. It is therefore of interest that both of the stars with
discrepant (
)
colours show excesses (S17 and S12;
Fig. 3), suggesting that a build-up of circumstellar material due to
enhanced mass-loss affects their near-IR properties, such as is observed
in RSGC2-49 (D07).
![]() |
Figure 5:
H-R diagram showing the locations of the 8 RSGs for which accurate spectral classification was possible assuming
a distance to the cluster of
|
Open with DEXTER |
Following the methodology pioneered by F06, we utilise Monte Carlo
simulations, employing rotating stellar models (D07) to estimate the
initial mass of the cluster from the number of RSGs currently
present. For ages of 16(20) Myr this yields masses of
.
If our second tier of RSG
candidates are confirmed as cluster members, the mass estimate would
increase by
30%, noting that this would not be altered by
increasing the distance to a maximum of 7 kpc (as implied by the
reddening). Finally, since stellar evolutionary codes predict
a spread in intrinsic RSG luminosities for even co-eval clusters, the observed range of
log (L
/
)
4.5-4.8 for cluster members should not be interpreted as
implying non-coevality.
4 Discussion and concluding remarks
With an age of 16-20 Myr and a total mass of
2-
,
RSCG3 appears to be a close counterpart
to RSGC1 and 2 (D08), while the properties of the constituent stars in
terms of spectral types, luminosities and circumstellar environments are
also directly comparable to the members of those clusters. As such RSGC3
belongs to an increasing population of hitherto unsuspected young massive clusters
within the Galaxy. With ages ranging from 2-3 Myr for the Arches to 16-20 Myr for
RSGC2 and 3 they also provide a fertile testbed for constraining the lifecycle of
stars of
and above.
Additionally, the proximity of RSGC3 to both RSGC1 and 2 - projected distances of pc and
pc
respectively for d=6 kpc - at the base of Scutum-Crux
arm (
)
provides strong support for the hypothesis that this region has
been subject to a recent burst of star formation (Garzón
et al. 1997, D07), yielding an extended stellar cluster complex such as those observed
in external galaxies such as M51 (Bastian et al. 2005). If correct, the
``starburst'' has yielded a total of >
of stars
just considering the 3 clusters. However, D07 report
the presence of an additional population of RSGs in the vicinity of RSGC2, while
Garzón et al. (1997) and López-Corredoira et al.
(1999) also report a significant ``diffuse'' field population
of cool supergiants within the region delineated by RSGC1-3, suggesting that the true total may
be significantly higher.
Consideration of the near-IR images of RSGC1-3 emphasises this
possibility; the clusters are only identifiable as such due to their
significant RSG populations, as no other overdensity of (less evolved) stars
is apparent. However, such stars
are intrinsically short lived and consequently rare, and so only signpost the
location of clusters for which the (unevolved) stellar population is
unresolvable against the field for a
narrow range of cluster masses and ages. For instance, for clusters
with masses of the order of
(so comparable to the Orion cluster) one would
only expect the presence of
1-3 RSGs at any given epoch on statistical grounds,
suggesting that such clusters would be difficult to identify in a near-IR
survey (a problem even afflicting more massive clusters for ages <
Myr; D07).
Likewise, for clusters with ages of over 20 Myr, while one would expect a large number of red evolved stars to be present, their lower intrinsic luminosity will make them more difficult to discriminate against the field in any search for cluster candidates. Furthermore, the rapid dissolution of clusters due to ejection of the intercluster medium by stellar winds and SNe (Goodwin & Bastian 2006) exacerbates this problem, reducing the spatial density of any RSGs within clusters. An additional result of this process is that the velocity dispersion of cluster members will increase, in turn magnifying the uncertainty in the identification of a physical association via kinematic means.
Therefore, while the presence of RSGC1-3 points to an episode of
enhanced star formation 10-20 Myr ago, from the methodology employed in this
work it is difficult to determine whether there was significant activity
before this date or indeed what the total stellar yield of this ``starburst''
was. An analogous
argument applies to determining the star formation history for ages <Myr, although the lack of Giant
H II regions within this putative complex (Conti & Crowther
2004) implies that no massive clusters are currently forming.
We caution that these limitations will manifest themselves in any other near-IR searches for stellar clusters in regions of the disc with a high (projected) stellar density, likely leading to significant incompleteness in any Galactic cluster mass function determined via such a methodology.
4.1 Association with X-ray binaries and post SNe compact objects
Given a potential SNe rate of one every 40-80 kyr (F06), and the
likely association of two pulsars with RSGC1 (F06, Gotthelf & Halpern
2008) we examined the
catalogues of Liu et al. (2006) and Bird et al. (2007) to
search for any relativistic sources associated with RSGC3, but found
none. Motivated by the hypothesis
that RSGC1-3 delineate a star forming complex, we extended this
search to this region (
,
), but again found
no accreting X-ray binaries within it for a
distance of
kpc. Given the increasing evidence for a high
binary fraction amongst massive stars (e.g., Clark et al. 2008), the absence of any Be/X-ray binaries -
systems consisting of a B0-3 V-IIIe primary and a neutron star accretor - is surprising, since one would expect them to
be active at such an epoch (10-20 Myr; e.g., Portegies Zwart &
Verbunt 1996). Nevertheless, one might suppose that a combination
of their transient nature plus a SNe kick
sufficient to either disrupt or rapidly eject a surviving binary from
the complex may explain their lack of detection.
However, we note with interest the location of the Anomalous X-ray
Pulsar (AXP) AX J1841.3-0455;
,
.
Durant & van Kerkwijk (2006) estimate a lower limit of
>
kpc to AX J1841.3-0455, while
Vasisht & Gotthelf (1997) provide an upper limit of 7 kpc
from its association with the SNR Kes 73. Taken together
they raise the possibility that it could be physically associated
with the putative star formation complex, with AX J1841.3-0455
located
equidistantly between RSGC2 and 3, and directly within the
region identified by Garzón et al. (1997) and López-Corredoira
et al. (1999) as showing a significant overdensity of ``field'' RSGs.
The expected rate of SNe for such a complex would be fully
consistent with the relative youth expected of magnetars (Thompson
et al. 2000), while SGR 1900+14 demonstrates that
their progenitors can have masses as low as
(Clark
et al. 2008; Davies et al. in prep.), also consistent with the
current RSG population at the base of the Scutum Crux arm.
Definitively associating AX J1841.3-0455 with the same burst of star
formation that yielded RSG1-3 would imply a progenitor mass of <,
and
hence provide additional evidence that the hypothesis that high-mass stars are required to produce
magnetars is incorrect. Moreover, consideration of
SGR 1900+14, AX J1838.0-0655 (the young pulsar associated with RSGC1)
and potentially AX J1841.3-0455 suggests that despite having
progenitors of comparable mass (
;
Clark et al. 2008, D08, Davies et al. in prep.),
the surface magnetic fields of the resultant neutron stars can differ by over two orders of magnitude
(Gotthelf & Halpern 2008; Kouveliotou et al. 1999;
Vasisht & Gotthelf 1997), presumably reflecting differences
in the properties of their progenitors other than, or being directly dependant on,
stellar mass (such as magnetic field or rotational velocity).
Acknowledgements
J.S.C. acknowledges support from an RCUK fellowship, and thanks Sophie Allen for assistance in the preparation of Fig. 1, and Mike & Tessa Allen for their kind hospitality during the production of this paper. This research is partially supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) under grants AYA2008-06166-C03-03 and CSD2006-70. AZT-24 observations are made within an agreement between Pulkovo, Rome and Teramo observatories D.F. acknowledges support from NASA under award NNG 05-GC37G, through the Long-Term Space Astrophysics program and from NYSTAR under a Faculty Development Program grant. The UKIDSS project is defined in Lawrence et al. (2007). UKIDSS uses the UKIRT Wide Field Camera (Casali et al. 2007). The photometric system and calibration are described in Hewett et al. (2006) and Hodgkin et al. (2008), with pipeline processing and archiving described in Irwin et al. (in prep.) and Hambly et al. (2008). This paper makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Note added in proof. Shortly after acceptance we became aware of the co-discovery of this cluster by Alexander et al. (2009).
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All Tables
Table 1:
Summary of RSG candidates and their properties.
Top panel:
the core group of 15 photometrically selected stars
regarded as prime cluster candidates; second panel: likely cluster
members; third panel: stars of interest, as defined in Sect. 2 and fourth panel: likely foreground
RSG identified by spectroscopy and included for completeness.
Note that based on spectroscopy, we also consider it likely that star S16 is a cluster member and treat it as such in the text. Co-ordinates
and near-IR magnitudes are from 2MASS, with mid-IR (5-25
m)
magnitudes from the Galactic plane surveys of GLIMPSE/ Spitzer
(Benjamin et al. 2003) and the Midcourse Source
Experiment (MSX) (Egan et al. 2001). We also
provide the dereddened MSX (A-C) colour (adopting the
prescription of Messineo et al. 2005), which is
a diagnostic of emission from circumstellar dust (and hence proxy for
mass loss) and the spectral type of the stars, where available (Sect. 3).
Table 2: Summary of the stellar properties of the 8 RSGs for which accurate spectral classification was possible, assuming a cluster distance of 6 kpc.
All Figures
![]() |
Figure 1:
Near-IR |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Finding chart for RSGC3, with the stars listed in Table 1
indicated. The finder comprises a |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Colour magnitude plot for stars within
|
Open with DEXTER | |
In the text |
![]() |
Figure 4: Montage of low (Campo Imperatore; black) and medium resolution (UKIRT; red) spectra of selected photometric targets, revealing the prominent CO bandhead absorption. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
H-R diagram showing the locations of the 8 RSGs for which accurate spectral classification was possible assuming
a distance to the cluster of
|
Open with DEXTER | |
In the text |
Copyright ESO 2009
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