Fokker-Planck Models for M15 without a Central Black Hole: The Role of the Mass Function

TL;DR

This study uses Fokker-Planck models to show that the globular cluster M15 can be explained without a central black hole, emphasizing the importance of the mass function and stellar remnants in its dynamics.

Contribution

The paper presents new Fokker-Planck models that fit M15's observations without requiring a central black hole, highlighting the role of stellar mass function and remnants.

Findings

No central black hole needed to fit observations

Good fits achieved with as few as 1600 neutron stars

Massive white dwarfs contribute significantly to gravitational potential

Abstract

We have developed a set of dynamically evolving Fokker-Planck models for the collapsed-core globular star cluster M15, which directly address the issue of whether a central black hole is required to fit Hubble Space Telescope (HST) observations of the stellar spatial distribution and kinematics. As in our previous work reported by Dull et al., we find that a central black hole is not needed. Using local mass-function data from HST studies, we have also inferred the global initial stellar mass function. As a consequence of extreme mass segregation, the local mass functions differs from the global mass function at every location. In addition to reproducing the observed mass functions, the models also provide good fits to the star-count and velocity-dispersion profiles, and to the millisecond pulsar accelerations. We address concerns about the large neutron star populations adopted in our previous Fokker-Planck models for M15. We find that good model fits can be obtained with as few as 1600 neutron stars; this corresponds to a retention fraction of 5% of the initial population for our best fit initial mass function. The models contain a substantial population of massive white dwarfs, that range in mass up to 1.2 solar masses. The combined contribution by the massive white dwarfs and neutron stars provides the gravitational potential needed to reproduce HST measurements of the central velocity dispersion profile.