Dynamics of direct impact accretion in degenerate binary systems

Abstract

ABSTRACT We consider the gas dynamics in an accreting binary system of degenerate stars within the framework of the Newtonian approximation. In such a system, the accretion stream can impact the surface of a white dwarf (WD) or neutron star (NS) as a result of the very compact orbit. This causes a loss of angular momentum from the orbit and spin-up of the accretor. We construct an approximation for the specific angular momentum of the accreting matter, which goes to spin up the accretor and approximations for some other parameters of the system. It is shown that the obtained approximation of the specific momentum is qualitatively different from the widely used Keplerian formula. It should affect the boundary between scenarios of immediate tidal disruption and slow mass-loss of the donor in WD–WD and NS–NS binaries, as well as the time of stable mass transfer in the stripping scenario.