The Role of High-mass Stellar Binaries in the Formation of High-mass Black Holes in Dense Star Clusters

TL;DR

This study uses detailed simulations to explore how the initial binary fraction among high-mass stars influences the formation of high-mass black holes in dense star clusters, revealing that core-collapse black hole masses depend on binary fraction, but merger-produced black holes do not.

Contribution

It demonstrates that the initial binary fraction among high-mass stars affects the number and mass of black holes formed via stellar collapse, but not those formed through mergers, providing new insights into black hole demographics.

Findings

Black hole mass from core-collapse scales with binary fraction.

Number of merger-formed black holes is independent of binary fraction.

Total high-mass black hole production is dominated by mergers in old clusters.

Abstract

Recent detections of gravitational waves from mergers of binary black holes (BBHs) with pre-merger source-frame individual masses in the so-called upper mass-gap, expected due to (pulsational) pair instability supernova ((P)PISN), have created immense interest in the astrophysical production of high-mass black holes (BHs). Previous studies show that high-mass BHs may be produced via repeated BBH mergers inside dense star clusters. Alternatively, inside dense star clusters, stars with unusually low core-to-envelope mass ratios can form via mergers of high-mass stars, which then can avoid (P)PISN, but produce high-mass BHs via mass fallback. We simulate detailed star-by-star multi-physics models of dense star clusters using the Monte Carlo cluster evolution code, CMC, to investigate the role of primordial binary fraction among high-mass stars (>=15 Msun) on the formation of high-mass BHs. We vary the high-mass stellar binary fraction (fb_15_prime) while keeping all other initial properties, including the population of high-mass stars, unchanged. We find that the number of high-mass BHs, as well as the mass of the most massive BH formed via stellar core-collapse are proportional to fb_15_prime. In contrast, there is no correlation between fb_15_prime and the number of high-mass BHs formed via BH-BH mergers. Since the total production of high-mass BHs is dominated by BH-BH mergers in old clusters, the overall number of high-mass BHs produced over the typical lifetime of globular clusters is insensitive to fb_15_prime. We study the differences in the demographics of BH-BH mergers and their implications for the LIGO-Virgo-Kagra detections as a function of fb_15_prime.