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

Cristiano Ugolini

TL;DR

This work uses the SEVN population-synthesis framework to quantify how single-star winds, core-collapse SN physics, and binary evolution collectively sculpt the BBH mass spectrum, across a wide grid of metallicities, CE prescriptions, and PPISN models. The key finding is that the notable $32$–$37\,M_{\odot}$ bump arises from a mix of CE and SMT channels, with their dominance controlled by the CE efficiency $\alpha$; low $\alpha$ favors CE, while higher $\alpha$ favors SMT. Additionally, a top-heavy Larson IMF dramatically increases both the bump population and overall merger rate, without overturning the CE/SMT balance. These results suggest that gravitational-wave observations can constrain CE physics and initial mass-function characteristics, and they plan to integrate these insights into a broader semi-analytic framework that accounts for metallicity and star-formation histories.

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.

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

TL;DR

This work uses the SEVN population-synthesis framework to quantify how single-star winds, core-collapse SN physics, and binary evolution collectively sculpt the BBH mass spectrum, across a wide grid of metallicities, CE prescriptions, and PPISN models. The key finding is that the notable – bump arises from a mix of CE and SMT channels, with their dominance controlled by the CE efficiency ; low favors CE, while higher favors SMT. Additionally, a top-heavy Larson IMF dramatically increases both the bump population and overall merger rate, without overturning the CE/SMT balance. These results suggest that gravitational-wave observations can constrain CE physics and initial mass-function characteristics, and they plan to integrate these insights into a broader semi-analytic framework that accounts for metallicity and star-formation histories.

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 binaries across 15 metallicities, 3 core-collapse supernova models, 4 PPISN models, and 6 common-envelope (CE) prescriptions, for a total of 990 runs ( systems). Both single- and binary-star physics shape the BH mass distribution: single-star processes control the high-mass tail (), while binary evolution produces pile-ups in specific intervals. In particular, the bump at , 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.
Paper Structure (5 sections, 2 figures)

This paper contains 5 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Effect of BSE on the 35 Ms peak
  4. Effect of a top-heavy IMF
  5. Summary and conclusions

Figures (2)

  • Figure 1: Primary BH mass distribution of merging BBHs for different CE efficiencies $\alpha$. Upper row: $\alpha=0.5,1,3$; lower row: $\alpha=5,10$, and the always-stable MT case. Blue: SMT channel; purple: CE channel. Dashed: metal-poor ($Z\leq 5\times10^{-4}$); solid: full population. The black curve is the total over all the channels.
  • Figure 2: Primary BH mass distribution of merging BBHs for a Larson IMF and different CE efficiencies $\alpha=3,5,10$. Blue: SMT; purple: CE. Black solid: Larson IMF; black dashed: Kroupa IMF.