Populating the upper black hole mass gap through stellar collisions in young star clusters

TL;DR

This paper investigates how stellar collisions in young dense star clusters can produce black holes in the upper mass gap, challenging single star evolution predictions and potentially explaining massive black hole mergers observed by LIGO/Virgo.

Contribution

It demonstrates through simulations that stellar collisions can create black holes within or above the pair-instability gap, providing a new formation channel for massive black holes in clusters.

Findings

Up to 20% of BH progenitors experience stellar collisions.

Approximately 1% of BHs form within or above the pair-instability gap.

Collisional formation channels can produce massive BH mergers comparable to observed events.

Abstract

Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above $\sim 40-50\,M_{\odot}$ for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae. However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called "upper-mass gap" would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as $\sim20\%$ of all BH progenitors undergo one or more collisions prior to stellar collapse and up to $\sim1\%$ of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the "multiple-generation" merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by LIGO/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to pair-instability supernovae and briefly discuss the expected rate of these events and other electromagnetic transients.