Radial variation of the stellar mass functions in the globular clusters M15 and M30: clues of a non-standard IMF?

TL;DR

This study analyzes the radial variation of stellar mass functions in globular clusters M15 and M30, suggesting they may have formed with a non-standard initial mass function based on observational data and N-body simulations.

Contribution

It provides the first detailed comparison of observed mass function variations with dynamical models, proposing a non-standard initial mass function for these clusters.

Findings

Both clusters show similar radial mass function variations.

Dynamical models cannot fully reproduce observed mass functions at their current ages.

Clusters may have formed with a non-standard, bottom-light initial mass function.

Abstract

We exploit a combination of high-resolution Hubble Space Telescope and wide-field ESO-VLT observations to study the slope of the global mass function (alphaG) and its radial variation (alpha(r)) in the two dense, massive and post core-collapse globular clusters M15 and M30. The available data-set samples the clusters' Main Sequence down to 0.2 Msun and the photometric completeness allows the study of the mass function between 0.40 Msun and 0.75 Msun from the central regions out to their tidal radii. We find that both clusters show a very similar variation in alpha(r) as a function of clustercentric distance. They both exhibit a very steep variation in alpha(r) in the central regions, which then attains almost constant values in the outskirts. Such a behavior can be interpreted as the result of long-term dynamical evolution of the systems driven by mass-segregation and mass-loss processes. We compare these results with a set of direct N-body simulations and find that they are only able to reproduce the observed values of alpha(r) and alphaG at dynamical ages (t/trh) significantly larger than those derived from the observed properties of both clusters. We investigate possible physical mechanisms responsible for such a discrepancy and argue that both clusters might be born with a non-standard (flatter/bottom-lighter) initial mass function.