Merging White Dwarfs, Thermonuclear Supernovae, Other Transients

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MERGING WHITE DWARFS, THERMONUCLEAR SUPERNOVAE, AND OTHER TRANSIENTS The merger of two white dwarfs can lead to a spectacular transient -- a thermonuclear supernova or accretion induced collapse to a neutron star -- or the formation of strange objects -- such as the R CrB helium giant stars. We recently realized that mergers of two white dwarfs made of carbon and oxygen might well produce most thermonuclear supernovae, and that this would, contrary to the current paradigm, likely happen independently of whether or not their total mass exceeds the maximum possible, Chandrasekhar mass. My proposal is to develop this new research direction, hoping to answer the following sets of questions: What triggers thermonuclear supernovae? From observations, can we distinguish ignition due to high density in a near-Chandrasekhar object, or due to high temperature in a sub-Chandrasekhar one? Does it happen in a fast merger with another white dwarf, or during slow accretion from a companion? What are the outcomes of white-dwarf mergers? Using simulations, what properties do merger remnants have? What happens as remaining disks accrete and spins evolve? Will some ignite nuclear runaways? Might detonations follow? Would this produce thermonuclear supernovae? What other transients might one expect? What types of transients occur in Nature? Do the multitudes of different transients predicted by theory actually occur? Are there ones we failed to imagine? For supernovae, can we see the shock break out of the star? See ejecta interact with a companion, a disk, or its environment?