Evolution of Massive Main-Sequence Stars in Binary Population Synthesis. II. Consequences for Black Hole High-Mass X-ray Binaries and Binary Black Holes

Interactions and mass loss during the main-sequence (MS) phase of massive binary stars can significantly alter their subsequent evolution and the properties of the black holes (BHs) they ultimately produce. We use the rapid binary population synthesis code COMPAS, incorporating a new treatment of MS evolution that self-consistently tracks the convective core under arbitrary mass-loss or mass-gain histories. We explore the properties of black hole high-mass X-ray binaries (BH-HMXBs) and binary black holes (BBHs) formed under this new framework. We find that a more detailed treatment of MS evolution is necessary to reproduce the high BH masses observed in BH-HMXBs, which are otherwise absent under prescriptions that do not take the internal structure of MS stars into account. Matching the observed rates and orbital periods favors case A mass transfer that is more non-conservative than commonly assumed. We further find that mass transfer of helium-enriched material during case A mass transfer is common in massive binaries, with potential consequences for the structure of the accreting star. For the BBH population, the new MS framework alters the chirp-mass and mass-ratio distributions of detectable mergers, and case A mass transfer can make a substantial contribution to the BBHs formed through the stable mass transfer channel. These systems show a strong preference for near-equal mass ratios, broadly consistent with recent constraints from gravitational-wave observations.