HEAD-ON COLLISIONS OF WHITE DWARFS IN TRIPLE SYSTEMS COULD EXPLAIN TYPE Ia SUPERNOVAE

Type Ia supernovae (SNe Ia), thermonuclear explosions of carbon-oxygen white dwarfs (CO-WDs), are currently the best cosmological "standard candles," but the triggering mechanism of the explosion is unknown. It was recently shown that the rate of head-on collisions of typical field CO-WDs in triple systems may be comparable to the SNe Ia rate. Here we provide evidence supporting a scenario in which the majority of SNe Ia are the result of such head-on collisions of CO-WDs. In this case, the nuclear detonation is due to a well understood shock ignition, devoid of commonly introduced free parameters such as the deflagration velocity or transition to detonation criteria. By using two-dimensional hydrodynamical simulations with a fully resolved ignition process, we show that zero-impact-parameter collisions of typical CO-WDs with masses 0.5-1 M-circle dot result in explosions that synthesize Ni-56 masses in the range of similar to 0.1-1 M-circle dot, spanning the wide distribution of yields observed for the majority of SNe Ia. All collision models yield the same late-time (greater than or similar to 60 days since explosion) bolometric light curve when normalized by Ni-56 masses (to better than 30%), in agreement with observations. The calculated widths of the Ni-56-mass-weighted line-of-sight velocity distributions are correlated with the calculated Ni-56 yield, agreeing with the observed correlation. The strong correlation, shown here for the first time, between Ni-56 yield and total mass of the colliding CO-WDs (insensitive to their mass ratio), is suggestive as the source for the continuous distribution of observed SN Ia features, possibly including the Philips relation.