The influence of initial mass segregation on the runaway merging of stars

TL;DR

This study uses N-body simulations to examine how initial mass segregation affects the likelihood of runaway stellar mergers in dense star clusters, finding it insufficient to trigger such events under typical Milky Way conditions.

Contribution

It demonstrates that initial mass segregation alone does not promote runaway stellar mergers in dense star clusters with typical densities.

Findings

Initial mass segregation decreases relaxation time.

Mass segregation does not increase stellar collision rates.

Runaway merging is unlikely in typical Milky Way clusters.

Abstract

We have investigated the effect of initial mass segregation on the runaway merging of stars. The evolution of multi-mass, dense star clusters was followed by means of direct N-body simulations of up to 131.072 stars. All clusters started from King models with dimensionless central potentials of 3.0 <= W_0 <= 9.0. Initial mass segregation was realized by varying the minimum mass of a certain fraction of stars whose either (1) distances were closest to the cluster center or (2) total energies were lowest. The second case is more favorable to promote the runaway merging of stars by creating a high-mass core of massive, low-energy stars. Initial mass segregation could decrease the central relaxation time and thus help the formation of a high-mass core. However, we found that initial mass segregation does not help the runaway stellar merger to happen if the overall mass density profile is kept constant. This is due to the fact that the collision rate of stars is not increased due to initial mass segregation. Our simulations show that initial mass segregation is not sufficient to allow runaway merging of stars to occur in clusters with central densities typical for star clusters in the Milky Way.