A case for Case A: detailed look at binary black hole formation through stable mass transfer

TL;DR

This study models binary black hole formation via stable mass transfer across various metallicities, revealing key dependencies on initial conditions, stellar winds, and natal kicks, and providing detailed population predictions.

Contribution

It offers a comprehensive analysis of BBH mergers from the SMT channel using detailed binary models across multiple metallicities, highlighting the physics and population characteristics involved.

Findings

BBH mergers mainly originate from systems with P_ZAMS ≤ 10 days on the main sequence.

Without natal kicks, the SMT channel does not produce mergers above 0.2Z_sun due to stellar wind effects.

Primary BH mass strongly depends on metallicity, with mass ratios tending toward unity.

Abstract

In isolated binary evolution, binary black hole (BBH) mergers are generally formed through stable mass transfer (SMT) or common envelope evolution. In recent years, the SMT channel has received significant attention due to detailed binary models showing increased mass transfer stability compared to previous studies. In this work, we perform a full zero-age-main-sequence to compact object merger analysis using detailed binary models at eight metallicities between $10^{-4}Z_\odot$ and $2Z_\odot$ to self-consistently model the population properties of BBH mergers in the SMT channel, determined their progenitor initial conditional, and investigate the binary physics governing their formation and metallicity dependence. We use the population synthesis code POSYDON to determine the population of BBH mergers from SMT. Using its extended grids of MESA binary models, we determine the essential physics in the formation of BBH mergers. SMT produces BBH mergers predominantly from systems with $P_{ZAMS}\leq10$ days. In these systems, both the initial mass transfer between two stars and the subsequent interaction between the remaining star and the first-born BH take place while the respective donor star is on the main-sequence (Case A). We find a limited contribution from wider Case B/C systems. Without a natal kick, the SMT channel does not produce BBH mergers above $Z>0.2Z_\odot$ due to orbital widening from stellar wind mass loss. The primary BH mass distribution shows a strong dependence on metallicity, while the mass ratio prefers unity independent of metallicity due to mass ratio reversal. Additionally, the $\chi_{eff}$ distributions contain peaks at $\chi_{eff}=0$ and ~0.15 of which the former disappears at high metallicities. A mass-scaled natal kick leave this sub-population unchanged but introduce a low-mass, unequal mass ratio sub-population that merges due to their high eccentricity.