Direct Impact Accretors

Abstract

We discuss the onset of mass transfer between two white dwarfs, focussing upon how they can survive to emerge as the semi-detached, hydrogen-deficient binary stars known as the AM CVn stars. We find that the finite size of the accretor compared to the binary separation causes two important effects: first, the mass transfer stream can hit the accretor directly; second, the mass transfer is destabilised, an effect that can only be counteracted if significant torques act to lock the spin of the accretor to the binary orbit.