Reconciling 56ni Production in Type Ia Supernovae with Double Degenerate Scenarios

Binary white dwarf (WD) coalescence driven by gravitational waves or collisions in triple systems are potential progenitors of Type Ia supernovae (SNe Ia). We combine the distribution of 56Ni inferred from observations of SNe Ia with the results of both sub-Chandrasekhar detonation models and direct collision calculations to estimate what mass WDs should be exploding in each scenario to reproduce the observations. These WD mass distributions are then compared with the observed Galactic WD mass distribution and Monte Carlo simulations of WD-WD binary populations. For collisions, we find that the average mass of the individual components of the WD-WD binary must be peaked at ≈ 0.75M⊙, significantly higher than the average WD mass in binaries or in the field of ≈ 0.55−0.60M⊙. Thus, if collisions indeed produce a large fraction of SNe Ia, then a mechanism must exist that favors large mass WDs. In particular, collisions between WDs of average mass must be highly suppressed. For sub-Chandrasekhar detonations, we find that the average mass of the exploding WDs must be peaked at ≈ 1.1M⊙, consistent with the average sum of the masses in WD-WD binaries. This interesting similarity should be tested by future calculations of the 56Ni yield from double degenerate mergers. These models may also explain why SNe Ia are on average dimmer in early-type hosts: in old environments binaries evolve too quickly to have mergers between two high mass WDs at current times. As future simulations explore the 56Ni yield over a wider range of parameters, the general framework discussed here will be an important tool for continuing to assess double degenerate scenarios. Subject headings: nuclear reactions, nucleosynthesis, abundances — supernovae: general — white dwarfs