Intermediate-mass Black Holes from High Massive-star Binary Fractions in Young Star Clusters

TL;DR

This study uses N-body simulations to show that young star clusters with high binary fractions and low metallicity can produce black holes within the pair-instability mass gap and intermediate-mass black holes through dynamical interactions.

Contribution

It demonstrates that dynamical interactions in young star clusters can naturally produce black holes in the upper-mass gap and intermediate-mass black holes, especially at low metallicities.

Findings

Clusters with high binary fractions form more massive black holes.

Black holes in the pair-instability gap can form dynamically.

Intermediate-mass black holes are often produced in these environments.

Abstract

Black holes formed in dense star clusters, where dynamical interactions are frequent, may have fundamentally different properties than those formed through isolated stellar evolution. Theoretical models for single star evolution predict a gap in the black hole mass spectrum from roughly $40-120\,M_{\odot}$ caused by (pulsational) pair-instability supernovae. Motivated by the recent LIGO/Virgo event GW190521, we investigate whether black holes with masses within or in excess of this "upper-mass gap" can be formed dynamically in young star clusters through strong interactions of massive stars in binaries. We perform a set of $N$-body simulations using the CMC cluster-dynamics code to study the effects of the high-mass binary fraction on the formation and collision histories of the most massive stars and their remnants. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants.