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

TL;DR

This study presents a simulation showing that the merger of two equal-mass white dwarfs around 0.9 solar masses can produce an underluminous type Ia supernova similar to 1991bg-like events, addressing previous modeling challenges.

Contribution

It demonstrates for the first time that equal-mass white dwarf mergers can lead to sub-luminous supernovae, matching observed spectral and color features.

Findings

Simulation of equal-mass white dwarf mergers produces underluminous explosions.

Resulting spectra and colors closely resemble 1991bg-like supernovae.

Masses between 0.83 and 0.9 solar masses are critical for this outcome.

Abstract

Type Ia supernovae (SNe Ia) 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 1991-bg-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 an underluminous explosion, though at the expense of requiring a single common-envelope phase, and component masses of ~0.9 M_sun. 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. While mass ratios can be slightly less than one and still produce an underluminous event, the masses have to be in the range 0.83-0.9 M_sun.