Letter to the Editor Evolution of white dwarf binaries: supersoft X-ray sources and progenitors of type Ia supernovae

The discovery of supersoft X-ray sources, described as massive white dwarfs burning accreted matter from their (more massive) companions, opens a new channel to type Ia supernovae (SNe Ia). According to conventional binary evolutionary theory, if the mass ratio of the donor to the white dwarf exceeds a critical value, the mass transfer becomes unstable, and a common envelope will be formed. However, recent calculations by Hachisu, Kato & Nomoto (1996) suggest a strong wind to stabilize the mass transfer when the mass transfer rate is higher than a certain value. Adopting the strong wind assumption,wehave performed evolutionary calculations ofwhite dwarf binaries, in which mass transfer occurs through Rochelobe overflow, to search for the progenitors of SNe Ia. We find that there are two types of systems that can produce SNe Ia. One is close binaries with ∼ 2 to ∼ 3.5M main-sequence or subgiant companions, and the initial orbital period of several tenth of a day to several days. The other is wide binaries with low-mass (∼ 1M ) red giant companions and long initial orbital period (tens to hundreds of days). The derived birth rate of SNe Ia in the Galaxy is roughly consistent with the observed one.