1. J.S. Clark 1, I. Negueruela 2, P.A. Crowther 3, S. Goodwin 4 and L. J. Hadfield 3 1 University College London, 2 Universidad de Alicante, 3 University Of Sheffield, 4 University of Wales, Cardiff COLOUR-MAGNITUDE DIAGRAMS  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. 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). 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 10 5 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. W243 – Spectroscopic monitoring Optical MOS / MXU Spectroscopy W243 – Spectroscopic monitoring 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. 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. INTRODUCTION  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.  In the Milky Way massive clusters are rare. The most massive examples known have 10 4 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.  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. VLT / I SSAC NTT / EMMI NTT / SUSI2 ESO 1.5m Tel/Instr VLT / UVES NTT / SOFI NTT / EMMI VLT / FORS1 VLT / NACO VLT / FORS2 VLT / I SSAC NTT / SOFI VLT / UVES VLT / MIDI 3.6m/ TIMMI ATCA Near IR Spectroscopy06/2002 Red Spectroscopy06/2002 UBVRI Photometry08/2001 Far IR Spectroscopy06/2001 ObservationDate 04/2004 Near IR Spectroscopy06/2003 Red Spectroscopy06/2003 VRI Imaging04/2003 Near IR A/O Imaging04/2003 06/2004 H-band Spectroscopy06/2004 Narrow-band 2µm Imaging06/2004 07/2004 Interferometry06/2004 Mid IR Imaging06/2004 3 – 20cm Imaging03/2003 OngoingVLT /VISIRMid IR Observations - 200 photometrically observed members are each more massive than 30 M  - Observed Stellar mass, M obs = 6  10 3 M  - Assuming a Kroupa IMF, M TOT = 10 5 M  - Presence of late O supergiants imply an upper age limit of ~ 5Myrs - Presence of WR Stars imply a lower age limit of > 3.5Myrs - Presence of hypergiants expected at ~ 4Myrs RESULTS 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. A SUPER STAR CLUSTER? 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. - Using the spectroscopically observed supergiant branch - Non-standard extinction Law.  DISTANCE  MASS - 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. - 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.  AGE  Wd1 is by far the most massive Galactic cluster.  Yellow Hypergiants and OB supergiants suggest d= 2.5 kpc.  Wd1 ~ 3.5–5Myr  Wd1 is very compact with half the observed population within a 25” radius, 0.3pc at 2.5kpc  The central density is estimated to be greater than 3  10 5 M  pc -3, such that Wd1 is 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.