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

Abstract

We present N-body simulations of intermediate-mass (3000–4000 M⊙) young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z⊙), 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 Z⊙) are enabled to form massive stellar black holes (MSBHs, with mass ≥25 M⊙) 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 Z⊙, 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 M⊙) 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.