High-resolution simulations of the head-on collision of white dwarfs

The direct impact of white dwarfs has been suggested as a plausible channel for Type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centres, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curve sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper, we address some of the possible sources of these differences, presenting the results of high-resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of Ni-56. To that purpose, we use a 2D axisymmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of Ni-56, ranging from 0.1 to 1.1 M-circle dot, compatible not only with normal-branch Type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass function of white dwarfs favours collisions among 0.6-0.7 M-circle dot objects, leading to subluminous events.