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 steady models for various accretion rates and white dwarf masses. We confirm that these are thermally stable only when the accretion rate is higher than similar to 10(-7) M(circle dot) yr(-1). We show that recent models of quiescent "surface hydrogen burning'' for a much wider range of accretion rates result from the use of too large a zone mass in the outer part of the models; hydrogen burning must occur in a much thinner layer. A comparison of the positions on the H-R 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 for the progenitors of Type Ia supernovae are discussed.