Black Hole Accretion and Spin-up Through Stellar Collisions in Dense Star Clusters

TL;DR

This study investigates how stellar collisions in dense star clusters influence black hole properties, especially spins, through simulations, revealing significant spin-up effects and their dependence on accretion physics, which impact gravitational-wave signals.

Contribution

It introduces a detailed analysis of black hole spin-up via stellar collisions in dense clusters using Monte Carlo models, highlighting the importance of accretion physics in spin evolution.

Findings

Up to 60% of black hole mergers are affected by stellar collisions.

Approximately 40% of merging black holes could have spins > 0.2 due to collisions.

Young clusters can produce black holes with high spins through accretion from massive stars.

Abstract

Dynamical interactions in dense star clusters could significantly influence the properties of black holes, leaving imprints on their gravitational-wave signatures. While previous studies have mostly focused on repeated black hole mergers for spin and mass growth, this work examines the impact of physical collisions and close encounters between black holes and (non-compact) stars. Using Monte Carlo N-body models of dense star clusters, we find that a large fraction of black holes retained upon formation undergo collisions with stars. Within our explored cluster models, the proportion of binary black hole mergers affected by stellar collisions ranges from $10\%$ to $60\%$. If all stellar-mass black holes are initially non-spinning, we find that up to $40\%$ of merging binary black holes may have components with dimensionless spin parameter $\chi\gtrsim 0.2$ because of prior stellar collisions, while typically about $10\%$ have spins near $\chi = 0.7$ from prior black hole mergers. We demonstrate that young star clusters are especially important environments as they can produce collisions of black holes with very massive stars, allowing significant spin up of the black holes through accretion. Our predictions for black hole spin distributions from these stellar collisions highlight their sensitivity to accretion efficiency, underscoring the need for detailed hydrodynamic calculations to better understand the accretion physics following these interactions.