Conditions For Successful Helium Detonations In Astrophysical Environments

TL;DR

This study investigates the conditions necessary for helium detonations in astrophysical environments, revealing that detonations can occur in lower-mass white dwarfs than previously thought, with incomplete burning and different nucleosynthesis outcomes.

Contribution

The paper provides the first systematic analysis of helium detonation initiation conditions, including critical length scales and implications for low-mass white dwarf explosions.

Findings

Helium detonations can be initiated in white dwarfs as low as 0.24 solar masses.

Initiation length scales are much smaller than the classical CJ length, enabling detonations in more systems.

Incomplete burning leads to different nucleosynthesis products, such as calcium and titanium, instead of nickel.

Abstract

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 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. A simple estimate of the length scales over which the total consumption of fuel will occur for steady-state detonations is provided by the Chapman-Jouguet (CJ) formalism. Our initiation lengths are consistently smaller than the corresponding CJ length scales by a factor of $\Sim 100$, 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 $^{40}$Ca, $^{44}$Ti, or $^{48}$Cr, rather than $^{56}$Ni, is the predominant burning product for many of these events. We anticipate that ...