A team of astronomers led by Dr Alicia Margarita Soderberg from the Harvard-Smithsonian Center for Astrophysics has announced the discovery of a new type of supernova named 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.
The newly discovered Type Iax, described in a paper accepted for publication in the Astrophysical Journal (arXiv.org version), is fainter and less energetic than Type Ia.
Although both types come from exploding white dwarfs, Type Iax supernovae may not completely destroy the white dwarf.
“A Type Iax supernova is essentially a mini supernova,” said first study author Dr Ryan Foley, also of the Harvard-Smithsonian Center for Astrophysics. “It’s the runt of the supernova litter.”
The 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 supernovae come from young star systems.
Based on a variety of observational data, the astronomers 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.
They 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,” said co-author Dr Mark Phillips from the Harvard-Smithsonian Center for Astrophysics.
Bibliographic information: Ryan Foley et al. 2013. Type Iax Supernovae: A New Class of Stellar Explosion. Accepted to ApJ; arXiv: 1212.2209