Black holes and core expansion in massive star clusters

A D Mackey,M I Wilkinson,G F Gilmore,M B Davies

doi:10.1111/j.1365-2966.2008.13052.x

Abstract

We present the results from realistic N-body modelling of massive star clusters in the Magellanic Clouds. We have computed eight simulations with N ~ 10^5 particles; six of these were evolved for at least a Hubble time. The aim of this modelling is to examine the possibility of large-scale core expansion in massive star clusters and search for a viable dynamical origin for the radius-age trend observed for such objects in the Magellanic Clouds. We identify two physical processes which can lead to significant and prolonged cluster core expansion: mass-loss due to rapid stellar evolution in a primordially mass segregated cluster, and heating due to a retained population of stellar-mass black holes. These two processes operate over different time-scales - the former occurs only at early times and cannot drive core expansion for longer than a few hundred Myr, while the latter typically does not begin until several hundred Myr have passed but can result in core expansion lasting for many Gyr. We investigate the behaviour of these expansion mechanisms in clusters with varying degrees of primordial mass segregation and in clusters with varying black hole retention fractions. In combination, the two processes can lead to a wide variety of evolutionary paths on the radius-age plane, which fully cover the observed cluster distribution and hence define a dynamical origin for the radius-age trend in the Magellanic Clouds. We discuss the implications of core expansion for various aspects of globular cluster research, as well as the possibility of observationally inferring the presence of a population of stellar-mass black holes in a cluster.

Highlights

As relatively simple objects which are integral to the study of many fundamental astronomical processes, massive star clusters are central to a wide variety of astrophysics over all scales – from star formation and stellar and binary evolution, through stellar exotica and variable stars, and the dynamics of self-gravitating systems, to galaxy formation and evolution, with implications for cosmology
Our main set of models is labelled Runs 1–4. These cover the extremes of the parameter space we are interested in investigating, spanned by black holes (BHs) retention fractions f BH = 0 and f BH = 1, and the pre-evolution durations TMS = 0 Myr and TMS = 450 Myr
In addition to our four primary runs, we performed several additional simulations in order to sample the parameter space more completely, and in particular verify that models with intermediate values of f BH and TMS exhibited evolution intermediate between that displayed by Runs 1–4

Summary

Introduction

As relatively simple objects which are integral to the study of many fundamental astronomical processes, massive star clusters are central to a wide variety of astrophysics over all scales – from star formation and stellar and binary evolution, through stellar exotica and variable stars, and the dynamics of self-gravitating systems, to galaxy formation and evolution, with implications for cosmology. It is relatively straightforward to turn our attention to the Large (LMC) and Small (SMC) Magellanic Clouds, which both possess extensive systems of star clusters with masses comparable to the Galactic globulars, but crucially of all ages: 106 τ 1010 yr. These two nearby galaxies are of fundamental importance to studies of star cluster evolution, because they are the only systems in which we can directly observe snapshots of cluster development over the last Hubble time using a sample of fully resolved objects

Objectives

Results

Conclusion