New kind of supernova discovered

Astronomers have discovered 25 examples of a new type of fainter and less energetic supernova.
The team of astronomers from Carnegie Institution reported the discovery of a new type of supernova called Type Iax.
Previously, supernovae were divided into either core-collapse or Type Ia categories. Core-collapse supernovae are the explosion of a star about 10 to 100 times as massive as our Sun. Type Ia supernovae are the complete disruption of a tiny white dwarf.
This new type, Iax, is fainter and less energetic than Type Ia. Although both types come from exploding white dwarfs, Type Iax supernovas may not completely destroy the white dwarf.

"A Type Iax supernova is essentially a mini supernova. It's the runt of the supernova litter," said lead author Ryan Foley, Clay Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA).
The research team identified 25 examples of the new type of supernova. None of them appeared in elliptical galaxies, which are filled with old stars. This suggests that Type Iax supernovas come from young star systems.

Based on a variety of observational data, the team concluded that a Type Iax supernova comes from a binary star system containing a white dwarf and a companion star that has lost its outer hydrogen, leaving it helium dominated. The white dwarf collects helium from the normal star.

Researchers could not determine what triggers a Type Iax. It's possible that the outer helium layer ignites first, sending a shock wave into the white dwarf. Alternatively, the white dwarf might ignite first due to the influence of the overlying helium shell.
It appears that in many cases the white dwarf survives the explosion, unlike in a Type Ia supernova where the white dwarf is completely destroyed.
The team calculated that Type Iax supernovae are about a third as common as Type Ia supernovae. The reason so few have been detected is that the faintest are only one-hundredth as bright as a Type Ia supernova.

The research will be published in The Astrophysical Journal.