The Initial Mass Function of Westerlund 1 Based on High Spatial Resolution HST Data

Abstract

Westerlund 1 is one of the most massive young star clusters in the Milky Way. We use precise photometry and astrometry from the Hubble Space Telescope to derive the fundamental properties of the cluster, including the initial mass function. Cluster membership probabilities are estimated using proper motions and photometry and 615 stars have membership probabilities above 90%. By comparing the cluster members with synthetic cluster models and Bayesian inference, we obtain the distance to the cluster 3:8 ± 0:2kpc, the age 14.7Myrs, and the average extinction Aks to the cluster 0:75 ± 0:01amg with an extinction variation 0:12 ± 0:02mag. The initial mass function is fit by a single power-law down to 1M☉ with an index of-1:58 ± 0:16, inconsistent with the "universal value" found in other low mass clusters (e.g. Orion), but consistent with the other starburst cluster (eg: Arches). We also find that the stellar evolution and atmosphere models do not match with our data in the pre-main sequence region. More improvement in the models are required in the future.