Letter to the Editor

A hidden population of Wolf-Rayet stars in the massive galactic cluster Westerlund 1 ^*

¹ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, England, UK
² Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
³ Dpto. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain

Corresponding author: J. S. Clark, jsc@star.ucl.ac.uk

Received: 2 October 2002
Accepted: 7 November 2002

Abstract

We report the discovery of a hitherto undetected population of Wolf-Rayet stars in the young galactic open cluster Westerlund 1. Optical spectroscopy of the cluster identified 11 such objects; provisional classification suggests that 6 are nitrogen rich (WN) and 5 carbon rich (WC). Including the previously identified Blue, Yellow & Red Super- & Hypergiants, Westerlund 1 clearly has a very rich population of massive post-Main Sequence objects. To date, the post-MS population of Westerlund 1 is significantly larger than that of any other galactic young open cluster – with the possible exception of the Arches – implying that it is potentially amongst the most massive young clusters yet identified in the Local Group.