Author(s): , , , , , ,
Institution(s): 1. IfA, U Hawaii, 2. STScI, 3. U Michigan, Dearborn, 4. UCLA, 5. Unaffiliated
Westerlund 1 (Wd 1) is one of the most massive young star clusters in the Milky Way disk and provides an excellent laboratory for studying star and cluster formation and early phases of stellar evolution. We present results from a new HST optical+IR study of Wd 1 that utilizes high-precision photometry and astrometry to measure the initial mass function (IMF), structure, and kinematics of this cluster. Proper motions are used to cleanly separate cluster members from contaminating field stars and four-band photometry is used to correct for spatially varying extinction. The final cluster catalog reaches stars as small as 0.15 Msun. We find that the resulting IMF shape is sensitive to the family of stellar models used to convert each star's observed brightness to mass and IMF shape errors are dominated by degeneracies in the distance, extinction, cluster age, and extinction law. We will present a comparison of Wd 1's IMF to that of Orion and other nearby, lower mass star clusters and discuss how the different initial conditions at different cluster masses influence the resulting star formation process.