Transcript Westerlund 1 : A Super-Star Cluster within the Milky Way

J.S.
1
Clark ,
1 University
I. Negueruela
College London,
2,
P.A. Crowther
2 Universidad
3,
4
S. Goodwin and L. J. Hadfield
3
de Alicante, 3 University Of Sheffield, 4 University of Wales, Cardiff
We present optical and IR photometric and spectroscopic observations of the young open cluster Westerlund 1 that indicate it is the first Super Star Cluster
identified in the Milky Way. Wd1 hosts a rich population of OB supergiants, Wolf-Rayet stars, Luminous Blue Variables, Yellow Hypergiants and Red Supergiants,
from which we infer an age of 3-5Myr. For an adopted Kroupa IMF we derive a mass of 105 M and radius of 0.3pc for an estimated distance of 2.5kpc. As such,
Wd1 is the most massive, and densest, young cluster in the Local Group, exceeding NGC3603 and the Arches cluster in the Milky Way and R136 in the LMC.
INTRODUCTION
RESULTS
 MASS
 Super Star Clusters (SSCs) represent the extreme in star forming environments.
Estimated to contain hundreds, maybe even thousands of massive stars, they can be
several orders of magnitudes more massive than normal open clusters, and are thought to
represent young, local analogues of Globular Clusters.
- 200 photometrically observed members are each more massive than 30 M
- Observed Stellar mass, Mobs = 6  103 M
- Assuming a Kroupa IMF , MTOT = 105 M
 Wd1 is by far the most massive Galactic cluster.
- This represents a lower limit to the mass of Wd1. Sample incompleteness is expected to
be significant, especially in the inner, crowded regions of the cluster.
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 In the Milky Way massive clusters are rare. The most massive examples known have
M and include NGC 3603, plus the Arches and Quintuplet clusters in the Galactic Centre.
 Westerlund 1 (Wd1) is a highly reddened (E(B-V)=4.5) open Galactic cluster (G339.55,
-0.40) whose massive star population remains elusive.
 DISTANCE
- Using the spectroscopically observed supergiant branch
- Non-standard extinction Law.
 The previous discovery of 11 Wolf-Rayet stars (Clark & Negueruela 2002) and the
presence of several yellow hypergiants (Westerlund 1987) hint that Wd1 could represent
a very massive cluster, since such stars represent only a short phase in the evolution of
massive stars.
 Yellow Hypergiants and OB supergiants suggest d= 2.5 kpc.
 AGE
- Presence of late O supergiants imply an upper age limit of ~ 5Myrs
- Presence of WR Stars imply a lower age limit of > 3.5Myrs
OBSERVATIONS
 Wd1 has been the target of an
ESO observing campaign since
2001.
 Observations presented in this
poster correspond to the first 3
observing runs,
- Spectroscopy
Intermediate resolution obtained
using NTT + EMMI.
8200 - 8900Å region was used for
spectral classification.
- Imaging
BVRI images were taken using
NTT + SUSI-2. Unfortunately
seeing conditions were relatively
poor.
- Presence of hypergiants expected at ~ 4Myrs
Date
Tel/Instr
Observation
06/2001
ESO 1.5m
Far IR Spectroscopy
08/2001
NTT / SUSI2
UBVRI Photometry
06/2002
NTT / EMMI
Red Spectroscopy
06/2002
VLT / I SSAC
Near IR Spectroscopy
03/2003
ATCA
3 – 20cm Imaging
 Wd1 is very compact with half the observed population within a 25” radius, 0.3pc at 2.5kpc
04/2003
VLT / NACO
Near IR A/O Imaging
 The central density is estimated to be greater than 3  105 M pc-3, such that Wd1 is
04/2003
VLT / FORS1
VRI Imaging
06/2003
NTT / EMMI
Red Spectroscopy
06/2003
NTT / SOFI
Near IR Spectroscopy
04/2004
VLT / UVES
W243 – Spectroscopic
monitoring
06/2004
NTT / SOFI
Narrow-band 2µm Imaging
06/2004
VLT / I SSAC
H-band Spectroscopy
06/2004
VLT / FORS2
Optical MOS / MXU
Spectroscopy
06/2004
3.6m/ TIMMI
Mid IR Imaging
06/2004
VLT / MIDI
Interferometry
07/2004
VLT / UVES
W243 – Spectroscopic
monitoring
Ongoing
VLT /VISIR
Mid IR Observations
SPECTROSCOPY
 Spectroscopy confirms that all cluster members identified are massive stars.
 53 stars which have been spectroscopically observed, form a bright supergiant
sequence from mid-O to late-M stars. In addition, 23 Wolf-Rayet stars are now known.
 6 A and F stars have been classified as hypergiants, each having luminosities of the
order of 5-8  L - some of the highest observed in the Milky Way.
 Wd1 ~ 3.5–5Myr
- For a cluster of this age and distance, the main sequence for stars ≤ 30 M would have
V>21mag, explaining the absence of a cluster main-sequence from the current CMD .
COLOUR-MAGNITUDE DIAGRAMS
A SUPER STAR CLUSTER?
predicted to go on to form an intermediate mass black hole (Portegies Zwart et al,
2004).
 Regions in the Local Group containing similar total masses are usually extended and consist
of several clusters, such as 30 Doradus in the LMC and NGC604 in M33.
 Due to its large mass and compact nature Wd1 is a Super Star Cluster.
Wd1
Colour magnitude diagram for a 5x5 ” field centred on Wd1. Objects with
spectroscopic classifications include: red supergiants (red), yellow hypergiants (yellow),
OB supergiants (blue) and WR stars (magenta).
 Colour-Magnitude diagrams (CMD) were constructed using V and I
band photometry since only the brightest members were detected in
the B-Band.
 NTT/SUSI2 Photometry is complete to V = 20 mag, i.e. 30 M.
 The well defined stellar sequence corresponds to a foreground
population towards the cluster.
 Cluster members are separated by (V-I) = 4 mag, resulting from an
intervening absorber.
 ~200 cluster stars cover a wide area of the CMD. This is the result
of observed members having evolved off the main sequence, and
partially due to variable extinction.
Wd1
Cluster radius and
density as a function of
mass. Local Group
Clusters (green) include
R136 and the Arches
and Quintuplet Clusters.
Extra Galactic clusters
(blue) include M82F,
NGC1569-A,NGC1705-1,
MGG-9, MGG-11.
How does Wd1 compare to other Super Star Clusters?
 It is the most massive open cluster observed in the Local Group, but is less massive than
SSCs observed in typical starburst galaxies such as M82.
 Wd1 bridges the gap between young clusters in the Milky Way and SSCs in
starbursts.
 Wd1 has a stellar density as great as any young cluster currently known, and so
represents an excellent candidate to search for merger events (W9 is a possible
unrelaxed merger remnant).
 The close proximity of Wd1 provides us with an example of a SSC, a thousand times
closer than any previously known.