Dynamics of binary black holes in young star clusters: the impact of cluster mass and long-term evolution

TL;DR

This study uses direct N-body simulations to explore how binary black hole formation and evolution differ in low- and high-mass star clusters, revealing distinct populations and dynamical processes affecting gravitational-wave sources.

Contribution

It provides the first detailed comparison of BBH formation channels in low- and high-mass star clusters, highlighting the impact of cluster mass on BBH properties and evolution.

Findings

Low-mass clusters produce mainly low-mass BBHs from binary evolution.

High-mass clusters host more massive BBHs driven by dynamical exchanges.

8% of BBHs in high-mass clusters have primary mass in the pair-instability gap.

Abstract

Dynamical interactions in dense star clusters are considered one of the most effective formation channels of binary black holes (BBHs). Here, we present direct $N-$body simulations of two different star cluster families: low-mass ($\sim{500-800}$ M$_\odot$) and relatively high-mass star clusters ($\ge{5000}$ M$_\odot$). We show that the formation channels of BBHs in low- and high-mass star clusters are extremely different and lead to two completely distinct populations of BBH mergers. Low-mass clusters host mainly low-mass BBHs born from binary evolution, while BBHs in high-mass clusters are relatively massive (chirp mass up to $\sim{100}$ M$_\odot$) and driven by dynamical exchanges. Tidal disruption dramatically quenches the formation and dynamical evolution of BBHs in low-mass clusters on a very short timescale ($\lesssim{100}$ Myr), while BBHs in high-mass clusters undergo effective dynamical hardening until the end of our simulations (1.5 Gyr). In high-mass clusters we find that 8\% of BBHs have primary mass in the pair-instability mass gap, all of them born via stellar collisions, while only one BBH with primary mass in the mass gap forms in low-mass clusters. These differences are crucial for the interpretation of the formation channels of gravitational-wave sources.