Carbon ignition in a rapidly accreting degenerate dwarf - A clue to the nature of the merging process in close binaries

Recent studies have suggested that the merging of two degenerate dwarfs composed of carbon and oxygen and of total mass larger than the Chandrasekhar limit occurs at a frequency comparable to that of Type I supernovae. The rate at which mass is transferred in the merging process is at present unknown, except that it must be less than some appropriate Eddington limit. It is found that, unless mass transfer occurs at a rate less than one-fifth of the Eddington limit for an isolated dwarf, carbon is ignited off-center, and ignition does not lead immediately to a deflagration. If carbon continues to burn quiescently until it is exhausted throughout the star, a neon-oxygen-magnesium white dwarf will be formed. If a Type I supernova is to follow from merging white dwarfs, a thick disk must be formed as an intermediate stage in the merging process, with transfer from the disk onto the central degenerate dwarf occurring at a rate sufficiently less than Eddington that a deflagration induced by carbon burning occurs. Thus, the outcome of the merging of two massive carbon-oxygen degenerate dwarfs is not trivially a Type I supernova explosion.