On the Prevalence of Early Mass Transfer for Very Massive Binaries

Abstract Common phases of mass transfer in massive stellar binaries are case A (during the donor’s main sequence) and case B (after the donor’s main sequence but before helium core depletion). Most stars see their radii significantly grow after the main sequence, making case B more common. However, very massive stars may already undergo significant expansion during the main sequence increasing the probability of case A mass transfer. We find that using convective boundary mixing informed by the width of the main sequence in 30 Doradus, case A mass transfer dominates for donor masses ≳75 M ⊙ . This is not the case without convective boundary mixing or in the stellar models commonly used in rapid binary population synthesis. Therefore, case A mass transfer may be more dominant than commonly assumed, with potential impact on rates of all post mass transfer binaries, from Wolf–Rayet-O-type binaries, to X-ray binaries and gravitational wave progenitors.