Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE])

Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE]):
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.