Supernovae were always thought to occur in two main varieties. But a team of astronomers¹ including Carnegie's Wendy Freedman, Mark Phillips and Eric Persson is reporting the discovery of a new type of supernova called Type Iax. This research has been accepted for publication in The Astrophysical Journal.

 

Previously², 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 dwarf³.

 

This new type, Iax, is fainter（微弱的） and less energetic than Type Ia. Although both types come from exploding white dwarfs⁴, Type Iax supernovas may not completely destroy the white dwarf. "A Type Iax supernova is essentially⁵ a mini supernova," says lead author Ryan Foley, Clay Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). "It's the runt of the supernova litter."

 

The research team--which also included Max Stritzinger, formerly⁶ of Carnegie--identified 25 examples of the new type of supernova. None of them appeared in elliptical galaxies⁷, 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 binary⁸ 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 aren't sure 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.

 

Either way, 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 calculates 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 closer we look, the more ways we find for stars to explode," Phillips said.

 

The Large Synoptic Survey Telescope could discover thousands of Type Iax supernovas over its lifetime.

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