Numerical simulations of off-center detonations in helium shells

We present results of two-dimensional simulations of detonations in helium shells, accreted above various C-O cores. Our calculations confirm results of one-dimensional calculations, showing that a detonation in a heavy helium layer of 0.1-0.3 M_⊙, must ignite a detonation in the core, leading to the total disruption of the star (Livne). It is found that the mechanism works at two different modes. For stars of total mass above 1.2 M_⊙, helium detonation develops into a double-detonation. This result is in agreement with the stationary analysis of Livne & Glasner. In smaller white dwarfs, the off-center detonation propagates around the core, leaving a strong shock wave which travels into the core, a shock that is not strong enough to develop into a double-detonation. When this shock wave converges toward the axis of symmetry, a carbon detonation is ignited off center (from the reverse side) and disrupts the entire core. Thus, as proposed by Livne, a detonation in a helium layer can ignite a detonation in cores of much lesser mass than the Chandrasekar mass. These results also raise an inherent argument against Branch & Nomoto's proposal for SN Ib.