Cole Holcomb, James Guillochon, Fabio De Colle, Enrico Ramirez-Ruiz
Several models for type Ia-like supernovae events rely on the production of a self-sustained detonation powered by nuclear reactions.In the absence of hydrogen, the fuel that powers these detonations typically consists of either pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that systematically determine the conditions required to initiate detonations in C/O material exist, but until now no analogous investigation of degenerate He matter has been conducted. We perform one-dimensional reactive hydrodynamical simulations at a variety of initial density and temperature combinations and find critical length scales for the initiation of He detonations that range between 1 -- $10^{10}$ cm. These sizes are consistently smaller than the corresponding Chapman-Jouguet (CJ) length scales by a factor of ~100, providing opportunities for thermonuclear explosions in a wider range of low mass white dwarfs (WDs) than previously thought possible. We find that virialized WDs with as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive WDs can be detonated if a sizable fraction of their mass is raised to a higher adiabat. That the initiation length is exceeded by the CJ length implies that certain systems may not reach nuclear statistical equilibrium within the time it takes a detonation to traverse the object. In support of this hypothesis, we demonstrate that incomplete burning will occur in the majority of He WD detonations and that $^{44}$Ti, rather than $^{56}$Ni, is the predominant burning product for many of these events. We anticipate that a measure of the quantity of $^{44}$Ti and $^{56}$Ni produced in a helium-rich thermonuclear explosion can potentially be used to constrain the nature of the progenitor system.
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http://arxiv.org/abs/1302.6235
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