Dynamics of stellar black holes in young star clusters with different metallicities - I. Implications for X-ray binaries

TL;DR

This study uses N-body simulations to explore how metallicity influences the formation and evolution of massive stellar black holes and their role in powering X-ray binaries in young star clusters.

Contribution

It demonstrates that dynamical exchanges significantly enhance the formation of massive black hole binaries, especially in low-metallicity environments, impacting X-ray binary populations.

Findings

Majority of MSBHs form via dynamical exchanges within 100 Myr.

A significant fraction of MSBHs are in binaries with wind accretion or RLO.

MSBHs can explain ultraluminous X-ray sources in low-metallicity clusters.

Abstract

We present N-body simulations of intermediate-mass (3000-4000 Msun) young star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1 Zsun), including metal-dependent stellar evolution recipes and binary evolution. 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. In particular, massive metal-poor stars (Z<=0.3 Zsun) are enabled to form massive stellar black holes (MSBHs, with mass >=25 Msun) through direct collapse. We find that three-body encounters, and especially dynamical exchanges, dominate the evolution of the MSBHs formed in our simulations. In SCs with Z=0.01 and 0.1 Zsun, about 75 per cent of simulated MSBHs form from single stars and become members of binaries through dynamical exchanges in the first 100 Myr of the SC life. This is a factor of >~3 more efficient than in the case of low-mass (<25 Msun) stellar black holes. A small but non-negligible fraction of MSBHs power wind-accreting (10-20 per cent) and Roche lobe overflow (RLO, 5-10 per cent) binary systems. The vast majority of MSBH binaries that undergo wind accretion and/or RLO were born from dynamical exchange. This result indicates that MSBHs can power X-ray binaries in low-metallicity young SCs, and is very promising to explain the association of many ultraluminous X-ray sources with low-metallicity and actively star forming environments.