Dynamics of direct impact accretion in degenerate binary systems

TL;DR

This paper models the gas dynamics in degenerate binary systems, revealing new approximations for angular momentum transfer that influence the understanding of mass transfer stability and binary evolution.

Contribution

It introduces novel approximations for the specific angular momentum of accreting matter, differing from Keplerian assumptions, impacting binary evolution scenarios.

Findings

New approximation for specific angular momentum differs from Keplerian formula.

Impacts boundary conditions for tidal disruption and mass loss.

Affects timing of stable mass transfer in binary systems.

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 momentumfrom the orbit and spin-up of the accretor. We construct approximations for the specific angular momentum of the accreting matter which goes to spin up the accretor and 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.