On the massive stellar population of the super star cluster Westerlund 1

¹ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK e-mail: jsc@star.ucl.ac.uk
² Dpto. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain
³ Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, England, UK
⁴ Department of Physics and Astronomy, University of Wales, Cardiff, CF24 3YB, Wales, UK

Received: 22 November 2004
Accepted: 16 January 2005

Abstract

We present new spectroscopic and photometric observations of the young Galactic open cluster Westerlund 1 (Wd 1) that reveal a unique population of massive evolved stars. We identify ~200 cluster members and present spectroscopic classifications for ~25% of these. We find that all stars so classified are unambiguously post-Main Sequence objects, consistent with an apparent lack of an identifiable Main Sequence in our photometric data to 20. We are able to identify rich populations of Wolf Rayet stars, OB supergiants and short lived transitional objects. Of these, the latter group consists of both hot (Luminous Blue Variable and extreme B supergiant) and cool (Yellow Hypergiant and Red Supergiant) objects – we find that half the known Galactic population of YHGs resides within Wd 1. We obtain a mean ~ 25 mag from the cluster Yellow Hypergiants, implying a Main Sequence turnoff at or below (O7 V or later). Based solely on the masses inferred for the 53 spectroscopically classified stars, we determine an absolute minimum mass of ~1.5 for Wd 1. However, considering the complete photometrically and spectroscopically selected cluster population and adopting a Kroupa IMF we infer a likely mass for Wd 1 of ~, noting that inevitable source confusion and incompleteness are likely to render this an underestimate. As such, Wd 1 is the most massive compact young cluster yet identified in the Local Group, with a mass exceeding that of Galactic Centre clusters such as the Arches and Quintuplet. Indeed, the luminosity, inferred mass and compact nature of Wd 1 are comparable with those of Super Star Clusters – previously identified only in external galaxies – and is consistent with expectations for a Globular Cluster progenitor.