Program: GS-2005B-Q-26

The determination of the initial mass function (IMF) for starburst regions and open clusters is still poorly known. The Galactic open cluster Westerlund 1 (Wd1) is a very suitable candidate for such study, since its stellar population can be resolved from low-mass stars to massive Wolf-Rayet (WR) objects. In this proposal we intend to use GMOS to do multi-object spectroscopy of evolved massive stars in Wd 1, including a number of new WR candidates. We expect to roughly double the number of WR stars detected in the cluster, constraining the high-mass end of the IMF. Moreover, the spectra will be analyzed in detail using state-of-the-art wind modeling that have already been applied by our group to another WR stars, allowing us to estimate the distance, reddenning and metalicity of the cluster.