The moment of core collapse in star clusters with a mass function

TL;DR

This study uses N-body simulations to analyze how the presence of massive stars influences the timing and indicators of core collapse in star clusters with a mass function, revealing that the collapse time scales inversely with the most massive star's mass.

Contribution

It introduces the concept of effective particle number to describe multi-mass cluster dynamics and links core collapse timing to the dynamical friction timescale of the most massive stars.

Findings

Core collapse time is inversely proportional to the most massive star's mass.

Core bounce becomes ambiguous when the most massive star exceeds 0.1% of the total cluster mass.

Total binding energy of hard binaries effectively indicates core collapse in massive-star-dominated clusters.

Abstract

Star clusters with multi-mass components dynamically evolve faster than those modeled with equal-mass components. Using a series of direct $N$-body simulations, we investigate the dynamical evolution of star clusters with mass functions, especially their core collapse time. Multi-mass clusters tend to behave like systems with a smaller number of particles, which we call the effective number of particles ($N_{eff}$) and for which $N_{eff} = M/m_{max}$ (here $M$ and $m_{max}$ are the total cluster mass and the mass of the most massive star in the cluster, respectively). We find that the time of core collapse is inversely proportional to the mass of the most massive star in the cluster and analytically confirm that this is because the core collapse of clusters with a mass function proceeds on the dynamical friction timescale of the most massive stars. As the mass of the most massive star increases, however, the core-collapse time, which is observed as a core bounce of the cluster core from the evolution of the core density or core radius, becomes ambiguous. We find that in that case the total binding energy of the hard binaries gives a good diagnosis for determining the moment of core collapses. Based on the results of our simulations, we argue that the core bounce becomes ambiguous when the mass of the most massive star exceeds 0.1\% of the total mass of the cluster.