Sub--Chandrasekhar Mass Models for Type IA Supernovae

Stellar evolution studies suggest that a common progenitor for Type Ia supernovae should be a carbon-oxygen white dwarf in the mass range 0.6 to 0.9 solar mass merging by gravitational radiation with a helium main-sequence star and accreting helium at a rate of several times 10^-8 solar mass/yr. Studies of symbiotic variables independently suggest a similar endpoint -- a carbon-oxygen white dwarf that, at the time of nuclear instability has accreted 0.15 to 0.20 solar mass of helium. In such systems helium burns in a detonation wave with such force that a detonation is also ignited in the interior of the dwarf. The entire system is disrupted and a bright display ensues, powered by the decay of Ni-56 synthesized in the explosion. We consider here the outcome of such events, calculating in detail their final composition, velocities, and light curves. The models do not fare badly compared with observations. The iron group nucleosynthesis is acceptable though there are, in some cases, substantial overproductions of Ti-44 and other isotopes of Ti, Cr, and V with respect to iron, suggesting that supernovae of this type are the origin in nature of these isotopes, but only part of the iron. The models tend to be subluminous compared with those based on carbon deflagration in white dwarfs near 1.4 solar mass. Interestingly, though peak luminosities are obtained that vary by up to a factor of 4, the shapes of the light curves and especially their post-peak decline rate are very similar. Future studies of radiation transport, spectral synthesis, and especially multidimensional hydrodynamics will be needed to clarify some of these properties.