Sub-Chandrasekhar Mass Models For Type Ia Supernovae

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  • 26 October 2010
Abstract
For white dwarfs accreting hydrogen or helium at rates in the range 1 to 5 x 10^(-8) Msun/yr, a variety of explosive outcomes are possible well before the star reaches the Chandrasekhar mass. The explosions begin as helium burning runaways that, for high enough ignition density, can lead to helium detonation. The possibility of helium deflagration is also considered. If helium detonation occurs in a spherically symmetric model with sufficiently fine zoning, the carbon-oxygen core also detonates, exploding the entire star. A grid of white dwarf masses and accretion rates is examined that explores each of these three possible outcomes - single helium detonation, "double detonation", and helium deflagration. The resulting nucleosynthesis, light curves and spectra are calculated. Empirically, a helium shell mass of at least 0.05 to 0.1 Msun is required to initiate a helium detonation. This mass has an appreciable effect on the dynamics of the explosion and causes peculiarities in the model spectra when compared to normal Type Ia supernovae. The light curves of the brighter explosions do obey a width-luminosity relation, but the relation differs quantitatively from what is observed. These sub-Chandrasekhar mass models seem best suited to making sub-luminous supernovae of a hitherto unusual kind. The models for helium deflagrations produce very faint transients that can be powered by radioactivities other than 56Ni. We conclude that, for the present observational sample, carbon detonation in sub-Chandrasekhar mass white dwarfs is a rare event. However, there could be a large number of faint transients corresponding to helium detonation or deflagration that are responsible for producing 44Ca in the sun. All our calculations are 1-D, however, and the need for multi-D simulations to clarify and quantify many of our results is emphasized.

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