Speaker
Description
Mass transfer is arguably one of the most important aspects in the evolution of binary stars, with the majority of binaries having at least one episode of mass transfer during their lifetime. Yet its efficiency — the fraction of the transferred mass that is accreted — remains plagued by uncertainties, despite dictating the outcomes of mass transfer and predicted populations of stripped-star binaries, Algol binaries, blue straggler stars, X-ray binaries, certain supernovae and gravitational wave sources. A common implementation of mass transfer in binary evolution models is rotationally limited accretion, which assumes mass accretion to be quenched once the accretor spins up to critical rotation, typically after accreting only ~5-10% of its initial mass. While physically motivated, the low efficiencies predicted by such a model are incompatible with the observed parameters of sdOB+Be binaries, a class of post-mass transfer systems that show evidence of conservative mass transfer. In contrast, observed populations of Algol binaries and WR+O binaries hint at lower efficiencies. This tension suggests that a single, fixed mass-transfer efficiency cannot explain different populations of binary stars. We investigate this variability and present an accretion model inspired by star-disk interactions, with a variable, mass-dependent efficiency emerging as a natural consequence. We also discuss the implications of such a framework for predicted populations of both stripped-star binaries and gravitational-wave sources.
