LATERALLY PROPAGATING DETONATIONS IN THIN HELIUM LAYERS ON ACCRETING WHITE DWARFS

Theoretical work has shown that intermediate mass (0.01M(circle dot) < M-He < 0.1M(circle dot)) helium shells will unstably ignite on the accreting white dwarf (WD) in an AM CVn binary. For more massive (M > 0.8M(circle dot)) WDs, these helium shells can be dense enough (> 5 x 10(5) g cm(-3)) 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 rather than an Eddington limited helium novae. The nature of the explosion (i.e., deflagration or detonation) remains ambiguous, is certainly density dependent, and likely breaks spherical symmetry. 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 laterally propagating detonation. These detonations are slower than the Chapman-Jouguet speed of 1.5x10(9) cm s(-1), but still fast enough at 0.9x10(9) cm s(-1) to go around the star prior to the transit through the star of the inwardly propagating weak shock. Our simulations resolve the subsonic region behind the reaction front in the detonation wave. The two-dimensional nucleosynthesis is shown to be consistent with a truncated one-dimensional Zeldovich-von Neumann-Doring calculation at the slower detonation speed. The ashes from the lateral detonation 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.