Collapse and explosion of white dwarfs. I - Precollapse evolution

Abstract

The long-term evolution of accreting carbon-oxygen white dwarfs in close binary systems is considered. Depending on the time of onset of mass accretion (for a given stellar mass), thermonuclear ignition happens when the star's center is either in the fluid or in the solid phase. In the last case, burning propagation should be slow, and previous carbon-oxygen separation is likely. It is shown that by considering different degrees of chemical separation (associated with different cooling times) diverse outcomes are possible: total collapse for maximum separation and off-center ignitions for partial chemical differentiation. The off-center ignitions might provide a mechanism for Type I supernova outbursts, and, by implying the explosive burning of varying amounts of the carbon-oxygen mixture, they might also explain the 'fast' to 'slow' SN I range. It is shown that the effects of solidification of the star's interior may be crucial in determining the fate of these systems, since they change the terms of the competition between thermonuclear ignition and collapse.