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

Type Ia supernovae are thought to result from thermonuclear explosions of carbon-oxygen white dwarf stars(1). Existing models(2) generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae(3). It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event(4,5), but hitherto simulations have failed to produce an explosion(6,7). 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 similar to 0.9M(circle dot). 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(circle dot) to 0.9M(circle dot).