Speaker
Description
A large fraction of stars interact with a close companion during their lifetime, during which the transfer of mass and angular
momentum shapes their evolution and final fate. Standard rotationally limited accretion models predict that accretors reach
critical rotation after gaining only a small fraction of their mass, severely suppressing further accretion. This is in tension with
observations of post-interaction systems that require substantial mass gain. We introduce a disc-mediated angular momentum
transport prescription for mass transfer onto stellar companions. This is based on a novel perturbative, analytic star-disc boundary
model that allows for continued mass inflow as the accretor approaches critical rotation. Furthermore, the model reproduces
previous numerical results in which perturbations to super-critical rotation result in negative torques exerted by the disc, extracting
excess angular momentum while allowing continued mass inflow. We implement this mechanism in detailed binary evolution
calculations with differential rotation using mesa, and compute grids spanning primary mass, orbital period, and mass ratio.
Whereas rotationally limited models predict 𝛽eff ≲ 0.1, the disc model yields sustained mass inflow near critical rotation, with
effective mass-transfer efficiencies of 𝛽eff ∼ 0.4–1 across much of the parameter space. The resulting accretor properties are
broadly consistent with observed post-interaction sdOB+Be binaries. Disc-mediated angular momentum transport may therefore
represent a key missing ingredient in standard binary evolution models, with important implications for rapidly rotating stars
and compact-object progenitors.