Dynamical Masses of Young Star Clusters: Constraints on the Stellar IMF and Star-Formation Efficiency

Abstract

Many recent works have attempted to constrain the stellar initial mass function (IMF) inside massive clusters by comparing their dynamical mass estimates to the measured light. These studies have come to different conclusions, with some claiming standard Kroupa-type IMFs, while others have claimed extreme non-standard IMFs. However, the results appear to be correlated with the age of the clusters, as older clusters (>80 Myr) all appear to be well fit by a Kroupa-type IMF whereas younger clusters display significant scatter in their best fitting IMF. Here we show that this is likely due to the fact that young clusters are out of virial equilibrium and therefore cannot be used for such studies. Hence only the older clusters are suitable for IMF studies. Using only these clusters we find that the IMF does not vary significantly. The youngest clusters can be used instead to constrain the star-formation efficiency (SFE) within clusters. We find that the SFE varies between 20 and 60% and we conclude that approximately 60% of young clusters are unbound and will not survive for more than a few 10's of Myr (i.e. infant mortality).