Laterally Propagating Detonations in Thin Helium Layers on Accreting White Dwarfs

TL;DR

This paper investigates the properties of laterally propagating helium detonations on accreting white dwarfs, showing they can sustain a detonation that produces specific nucleosynthesis signatures, relevant for understanding certain supernovae.

Contribution

It provides the first detailed simulations of lateral helium detonation propagation in thin shells on white dwarfs, including reaction structure and nucleosynthesis outcomes.

Findings

Detonation propagates at subsonic speeds slower than Chapman-Jouguet.

Incomplete helium burning occurs due to rapid radial expansion.

Nucleosynthesis yields include Ti-44, Cr-48, Fe-52, and Ca-40.

Abstract

Theoretical work has shown that intermediate mass (0.01Msun<M_He<0.1Msun) Helium shells will unstably ignite on the accreting white dwarf (WD) in an AM CVn binary. For more massive (M>0.8Msun) WDs, these helium shells can be dense enough (5x10^5 g/cc) that the convectively burning region runs away on a timescale comparable to the sound travel time across the shell; raising the possibility for an explosive outcome. The nature of the explosion (i.e. deflagration or detonation) remains ambiguous. In the case of detonation, this causes a laterally propagating front whose properties in these geometrically thin and low density shells we begin to study here. Our calculations show that the radial expansion time of <0.1 s leads to incomplete helium burning, in agreement with recent work by Sim and collaborators, but that the nuclear energy released is still adequate to realize a self-sustaining detonation propagating laterally at slower than the Chapman-Jouguet speed. Our simulations resolve the subsonic region behind the front and are consistent with a direct computation of the reaction structure from the shock strength. The ashes are typically He rich, and consist of predominantly Ti-44, Cr-48, along with a small amount of Fe-52, with very little Ni-56 and with significant Ca-40 in carbon-enriched layers. If this helium detonation results in a Type Ia Supernova, its spectral signatures would appear for the first few days after explosion. (abridged)