Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass ∼0.9M⊙

Abstract

Type Ia supernovae are thought to result from thermonuclear explosions of carbon–oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of ∼0.9M⊙. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83M⊙ to 0.9M⊙.