Accreting white dwarf models for type I supernovae. I - Presupernova evolution and triggering mechanisms

Results are presented of numerical calculations of the presupernova evolution and triggering mechanisms of accreting white dwarf stars, which have been suggested as the progenitors of type I supernovae. The evolution of carbon-oxygen white dwarf models accreting helium in binary systems was computed using a Henyey-type hybrid method including both thermal and hydrodynamical equations. It is found that type I supernovae can be triggered by the off-center detonation of helium in systems with slow or intermediate accretion rates, or by the central carbon flash for slow or rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion rates due to the importance of initial mass in determining the mode of ignition. Attention is also given to possible mechanisms for the building up of the helium zone surrounding the white dwarf, including direct transfer of helium from the companion star and hydrogen shell burning by processes depending on the accretion rate.