The Intermediate-Mass Population in the Core of the R136 Star Cluster

Abstract

R136, the compact, luminous star cluster at the center of the 30 Doradus nebula, represents an extreme mode of star formation. Yet it is unique among these kinds of objects in that it can be resolved into individual stars and the star formation process probed through the stellar products. In a previous paper we reported on a study of the intermediate stellar mass population in R136 and derived a stellar initial mass function (IMF) for intermediate-mass stars in the region 0.5 to 4.7 pc from the center of the cluster. We have now obtained Hubble Space Telescope images designed to probe the stellar population in the core ( < 0.5 pc) of the cluster. We have measured the IMF for stars 5-15 M☉ at radii 0.11-0.34 pc, and for 4-15 M☉ at 0.34-0.46 pc. Beyond 0.46 pc we measure stars in mass bins from 2.8-15 M☉, as before. The slope of the stellar IMF beyond 0.5 pc is found to be -1.0 ± 0.1, which is within a few sigma of the value measured previously for the same annulus. The slopes of the IMFs become shallower at smaller radii, but the uncertainties do not rule out an IMF that is constant with cluster radius. The lower stellar mass limit is ≤5 M☉ in the cluster core. Thus, R136 is found to be similar to less concentrated OB associations even into 0.1 pc from the center of the cluster.