Head-on collisions of white dwarfs in triple systems could explain type Ia supernova

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 $0.5-1\,M_{\odot}$ result in explosions that synthesize $^{56}$Ni masses in the range of $\sim0.1-1\,M_{\odot}$, spanning the wide distribution of yields observed for the majority of SNe Ia. All collision models yield the same late-time ($>60$ days since explosion) bolometric light curve when normalized by $^{56}$Ni masses (to better than $30\%$), in agreement with observations. The calculated widths of the $^{56}$Ni-mass-weighted-line-of-sight velocity distributions are correlated with the calculated $^{56}$Ni yield, agreeing with the observed correlation. The strong correlation, shown here for the first time, between $^{56}$Ni 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.