A Stellar Role Reversal: Multiple Features in the Mass and Mass Ratio Distributions of Merging Binary Black Holes from Stable Mass Transfer

Observations of gravitational wave events have enabled the measurement of the merging binary black hole (BBH) mass function. This mass function encodes the physical interactions which shape the formation and evolution of BBHs. In this work we investigate how the stable mass transfer (SMT) channel of BBH formation imprints onto the BBH primary mass and mass ratio distributions. We use both an analytic framework and binary population synthesis to show how assumptions about mass transfer accretion efficiency and mass transfer stability affect the BBH mass distribution. Under the assumption of conservative mass transfer, we find that the SMT channel produces two observationally distinct subpopulations: a high primary mass, near equal mass ratio population formed through mass ratio reversal (MRR), and a low primary mass non-MRR subpopulation. The mass range where MRR occurs is determined by assumptions about binary SMT. In particular, we find that the stability criteria for mass transfer at different stellar evolutionary stages carve out complementary regions in the primary-mass--mass-ratio plane, separating the MRR and non-MRR populations into distinct peaks at high and low primary mass respectively. Our results imply that the physics of SMT creates distinct features in gravitational wave populations which current and near future gravitational wave detectors may be able to resolve.