Dynamics of stellar black holes in young star clusters with different metallicities - II. Black hole-black hole binaries

TL;DR

This study uses N-body simulations to explore how black hole binaries form and evolve in young star clusters with varying metallicities, revealing their properties, formation times, and potential merger rates relevant for gravitational wave detectors.

Contribution

It provides the first detailed dynamical modeling of BH-BH binary formation in young star clusters across different metallicities, incorporating metallicity-dependent stellar evolution.

Findings

BH-BH binaries are formed efficiently via dynamical exchanges.

More massive BHs and earlier formation in metal-poor clusters.

Most BH-BH binaries have large chirp masses and wide orbital periods.

Abstract

In this paper, we study the formation and dynamical evolution of black hole-black hole (BH-BH) binaries in young star clusters (YSCs), by means of N-body simulations. The simulations include metallicity-dependent recipes for stellar evolution and stellar winds, and have been run for three different metallicities (Z = 0.01, 0.1 and 1 Zsun). Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. We find that BH-BH binaries form efficiently because of dynamical exchanges: in our simulations, we find about 10 times more BH-BH binaries than double neutron star binaries. The simulated BH-BH binaries form earlier in metal-poor YSCs, which host more massive black holes (BHs) than in metal-rich YSCs. The simulated BH-BH binaries have very large chirp masses (up to 80 Msun), because the BH mass is assumed to depend on metallicity, and because BHs can grow in mass due to the merger with stars. The simulated BH-BH binaries span a wide range of orbital periods (10^-3-10^7 yr), and only a small fraction of them (0.3 per cent) is expected to merge within a Hubble time. We discuss the estimated merger rate from our simulations and the implications for Advanced VIRGO and LIGO.