Piling up in the darkness: Features of the BBH mass distribution from isolated binaries

TL;DR

This study models the impact of stellar evolution and binary processes on black hole mass distributions, revealing features like mass pile-ups and the influence of initial mass functions, to interpret gravitational wave observations.

Contribution

It provides a comprehensive analysis of how different stellar and binary evolution models shape the BBH mass distribution, including features like the 35 solar mass bump.

Findings

High-mass tail influenced by single-star physics.

Binary evolution causes specific mass pile-ups.

Top-heavy IMF increases merger rates.

Abstract

After the third LIGO--Virgo--KAGRA observing run, the number of detected binary black hole (BBH) mergers became sufficient to identify statistical features of the population. We explore how different prescriptions for the final fate of massive stars and key binary-evolution processes shape isolated binaries and their remnants. Using \textsc{sevn}, we evolved $10^{7}$ binaries across 15 metallicities, 3 core-collapse supernova models, 4 PPISN models, and 6 common-envelope (CE) prescriptions, for a total of 990 runs ($9.9 \times 10^{9}$ systems). Both single- and binary-star physics shape the BH mass distribution: single-star processes control the high-mass tail ($M_{\rm BH} \geq 45M_{\odot}$), while binary evolution produces pile-ups in specific intervals. In particular, the bump at $\sim 35 M_{\odot}$, often attributed to PPISNe, also emerges from binaries evolving only through stable mass transfer, without CE. Finally, we test a top-heavy IMF, finding it boosts merger numbers and alters the abundance of systems with a given primary BH mass.