2000A&A...362.1046L

Close binaries consisting of a main sequence star and a white dwarf are considered as candidates for Type Ia supernova progenitors. We present selfconsistent calculations of the time dependence of the structure of the main sequence star, the mass transfer rate, and the orbit by means of a binary stellar evolution program. We obtain, for the first time, a complete picture of the time evolution of the mass transfer rate in such systems. We find a long switch-on phase of the mass transfer, about 10⁶yr, during which nova outbursts should persist in all systems. Furthermore, we find that the white dwarfs can reach the Chandrasekhar mass only during the decline phase of the mass transfer, which may have consequences for the critical accretion rate for stationary nuclear burning on the white dwarf surface. In contrast to results based on simple estimates of the mass transfer rate in systems of the considered type, our results allow for the possibility that even systems with rather small initial white dwarf masses (∼0.7M_☉) may produce Type Ia supernovae, which then might originate from very rapidly rotating white dwarfs. We present results for two different metallicities, Z=0.02 and Z=0.001. We find that for systems with the lower metallicity, the mass transfer rates are on average five times larger than in comparable system at solar metallicity. This leads to a systematic shift of the supernova Ia progenitor population. Firstly, while for Z=0.02 - for our choice of white dwarf wind mass loss and mass accumulation rate - donor star initial masses in supernova progenitor systems are restricted to the range 1.6M_☉...2.3M_☉, they are in the interval 1.4M_☉...1.8M_☉ at low Z. Secondly, the initial white dwarf masses need, on average, to be larger by 0.2M_☉ at low Z in order to obtain a Chandrasekhar mass white dwarf. This metallicity dependences have very little effect on the progenitor life times, but may be responsible for a drop of the Type Ia supernova rate for low metallicity, and may introduce a Z-dependence in the properties of supernovae which stem from close main sequence star + white dwarf systems. We estimate the X-ray luminosities of the computed systems, and investigate their donor star and orbital properties. We find the donor stars to be underluminous by up to one order of magnitude, and more compact than normal main sequence stars. In general, our systems correspond well to observed close binary supersoft X-ray sources. We further derive the chemical and kinematical properties of the stellar remnants of our systems after the explosion of the white dwarf, which may serve as a test of the viability of the considered Type Ia supernova scenario.