The critical mass ratio of double white dwarf binaries for violent merger-induced Type Ia supernova explosions

TL;DR

This paper refines the critical mass ratio for violent mergers of double white dwarfs leading to Type Ia supernovae, using hydrodynamical simulations, and assesses implications for supernova brightness and specific binary systems.

Contribution

The study provides a more stringent critical mass ratio for violent WD mergers based on SPH simulations, considering initial conditions, and evaluates its impact on supernova brightness distribution and the fate of a specific binary system.

Findings

New critical mass ratio (qcr) derived from SPH simulations.

qcr is mainly affected by initial conditions like WD spin.

Final fate of the binary system Henize 2-428 is likely a sub-Chandrasekhar mass SN Ia.

Abstract

Mergers of carbon-oxygen (CO) white dwarfs (WDs) are considered as one of the potential progenitors of type Ia supernovae (SNe Ia). Recent hydrodynamical simulations showed that the less massive (secondary) WD violently accretes onto the more massive (primary) one, carbon detonation occurs, the detonation wave propagates through the primary, and the primary finally explodes as a sub-Chandrasekhar mass SN Ia. Such an explosion mechanism is called the violent merger scenario. Based on the smoothed particle hydrodynamics (SPH) simulations of merging CO WDs, we derived more stringent critical mass ratio (qcr) leading to the violent merger scenario than the previous results. We conclude that this difference mainly comes from the differences in the initial condition, synchronously spinning of WDs or not. Using our new results, we estimated the brightness distribution of SNe Ia in the violent merger scenario and compared it with previous studies. We found that our new qcr does not significantly affect the brightness distribution. We present the direct outcome immediately following CO WD mergers for various primary masses and mass ratios. We also discussed the final fate of the central system of the bipolar planetary nebula Henize 2-428, which was recently suggested to be a double CO WD system whose total mass exceeds the Chandrasekhar-limiting mass, merging within the Hubble time. Even considering the uncertainties in the proposed binary parameters, we concluded that the final fate of this system is almost certainly a sub-Chandrasekhar mass SN Ia in the violent merger scenario.