Thermal Stability of White Dwarfs Accreting Hydrogen-rich Matter and Progenitors of Type Ia Supernovae

We revisit the properties of white dwarfs accreting hydrogen-rich matter by constructing steady-state models, in which hydrogen shell burning consumes hydrogen at the same rate as the white dwarf accretes it. We obtain such steady-state models for various accretion rates and white dwarf masses. We confirm that these steady models are thermally stable only when the accretion rate is higher than ∼ 10M⊙ yr . We show that recently claimed “quiescent burning” in the “surface hydrogen burning” models at a much wider range of accretion rates results from the too large zone mass in the outer part of the models; hydrogen burning must occur in a much more superficial layer. A comparison of the positions on the HR diagram suggests that most of the luminous supersoft X-ray sources are white dwarfs accreting matter at rates high enough that the hydrogen burning shell is thermally stable. Implications on the progenitors of Type Ia supernovae are discussed. Subject headings: accretion – binaries: close – stars: evolution – novae, cataclysmic variables — supernovae: general – white dwarfs submitted to the Astrophysical Journal