Evidence for a bottom-light initial mass function in massive star clusters

We have determined stellar mass functions of 120 Milky Way globular clusters and massive Large Magellanic Cloud/Small Magellanic Cloud star clusters based on a comparison of archival Hubble Space Telescope photometry with a large grid of direct N-body simulations. We find a strong correlation of the global mass function slopes of star clusters with both their internal relaxation times and their lifetimes. Once dynamical effects are being accounted for, the mass functions of most star clusters are compatible with an initial mass function described by a broken power-law distribution N(m) similar to m(alpha) with break masses at 0.4 and 1.0 M-circle dot and mass function slopes of alpha(Low) = -0.3 for stars with masses m < 0.4 M-circle dot, alpha(High) = -2.30 for stars with m > 1.0 M-circle dot, and alpha(Med) = -1.65 for intermediate-mass stars. Alternatively, a lognormal mass function with a characteristic mass logM(C) = -0.36 and width sigma(C) = 0.28 for low-mass stars and a power-law mass function for stars with m > 1 M-circle dot also fit our data. We do not find a significant environmental dependence of the initial mass function on cluster mass, density, global velocity dispersion, or metallicity. Our results lead to a larger fraction of high-mass stars in globular clusters compared to canonical Kroupa/Chabrier mass functions, increasing the efficiency of self-enrichment in clusters and helping to alleviate the mass budget problem of multiple stellar populations in globular clusters. By comparing our results with direct N-body simulations, we finally find that only simulations in which most black holes are ejected by natal birth kicks correctly reproduce the observed correlations.

Automated summary

We determined the stellar mass functions of 120 Milky Way globular clusters and massive star clusters in the Magellanic Clouds. The mass functions of the star clusters are correlated with their internal relaxation times and lifetimes. When considering dynamical effects, most star clusters have mass functions that follow a broken power-law or a lognormal distribution. We also found that the initial mass function does not depend significantly on cluster mass, density, velocity dispersion, or metallicity. Our results suggest a larger fraction of high-mass stars in globular clusters compared to canonical mass functions, which affects the self-enrichment process and helps explain the presence of multiple stellar populations.