The Impact of Angular Momentum Loss on the Outcomes of Binary Mass Transfer

We use the rapid binary population synthesis code COMPAS to investigate commonly used prescriptions for the determination of mass transfer stability in close binaries and the orbital separations after stable mass transfer. The degree of orbital tightening during non-conservative mass transfer episodes is governed by the poorly-constrained angular momentum carried away by the ejected material. Increased orbital tightening drives systems towards unstable mass transfer leading to a common envelope. We find that the fraction of interacting binaries that will undergo only stable mass transfer throughout their lives fluctuates between a few and $\sim 20\%$ due to uncertainty in the angular momentum loss alone. If mass transfer is significantly non-conservative, stability prescriptions that rely on the assumption of conservative mass transfer under-predict the number of systems which experience unstable mass transfer and stellar mergers. This may substantially impact predictions about the rates of various transients, including luminous red novae, stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing compact binaries.