Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs

Type Ia supernovae are believed to be white dwarfs disrupted by a thermonuclear explosion. Here we investigate the scenario in which a rather low-mass, carbon-oxygen (C + O) white dwarf accumulates helium on its surface in a sufficient amount for igniting a detonation in the helium shell before the Chandrasekhar mass is reached. In principle, this can happen on white dwarfs accreting from a non-degenerate companion or by merging a C + O white dwarf with a low-mass helium one. In this scenario, the helium detonation is thought to trigger a secondary detonation in the C + O core. It is therefore called the “double-detonation sub-Chandrasekhar” supernova model. By means of a set of numerical simulations, we investigate the robustness of this explosion mechanism for generic 1-M models and analyze its observable predictions. Also a resolution dependence in numerical simulations is analyzed. Hydrodynamic simulations of the double-detonation sub-Chandrasekhar scenario are conducted in two and three spatial dimensions. The propagation of thermonuclear detonation fronts, both in helium and in the carbon-oxygen mixture, is computed by means of both a level-set function and a simplified description for nuclear reactions. The decision whether a secondary detonation is triggered in the white dwarf’s core or not is made based on criteria given in the literature. In a parameter study involving different initial flame geometries for He-shell masses of 0.2 and 0.1 M (and thus 0.8 and 0.9 M of C + O), we find that a secondary detonation ignition is a very robust process. Converging shock waves originating from the detonation in the He shell generate the conditions for a detonation near the center of the white dwarf in most of the cases considered. Finally, we follow the complete evolution of three selected models with 0.2 M of He through the C/O-detonation phase and obtain 56Ni-masses of about 0.40 to 0.45 M . Although we have not done a complete scan of the possible parameter space, our results show that sub-Chandrasekhar models are not good candidates for normal or sub-luminous type Ia supernovae. The chemical composition of the ejecta features significant amounts of 56Ni in the outer layers at high expansion velocities, which is inconsistent with near-maximum spectra.