Evolutionary models for double helium white dwarf mergers and the formation of helium-rich hot subdwarfs

Recent surveys have demonstrated the existence of several short-period binary systems containing two white dwarfs. Following orbital decay by gravitational-wave radiation, such binaries are expected to merge at a rate of two or three per thousand years per galaxy. The consequences of such a merger depend on the individual white dwarf masses, but are believed to include helium-rich subdwarfs, R CrB stars, extreme helium stars and also AM CVn systems and possibly Type Ia supernovae. Whilst the hydrodynamics of the merger process remains difficult to compute, it is possible to compute the evolution of a double white dwarf merger following the destruction of one component. In this paper, we describe the evolution following the merger of two helium white dwarfs. We examine three sets of assumptions concerning the distribution of debris material between a disc and a corona. Our results demonstrate that a model comprising both fast accretion to form a (hot) corona and slow accretion from a (cold) debris disc can reproduce the observed distribution of helium-rich subdwarfs in terms of their surface temperatures, gravities, nitrogen and carbon abundances.