Pasadena, CA— Supernovae were always thought to occur in two main varieties. But a team of astronomers 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 and is available online.

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,” 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, formerly of Carnegie–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 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.

This research was supported by a Clay Fellowship, the NSF, Gary & Cynthia Bengier, the Richard & Rhoda Goldman Fund, the Christopher R. Redlich Fund, the TABASGO Foundation, Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the University of California, the Sylvia & Jim Katzman Foundation, the Danish Agency for Science and Technology and Innovation, the Millennium Center for Supernova Science with input from ‘Fondo de Innovacion para la Competitividad, del Ministerio de Economıa, Fomento y Turismo de Chile,’ and CONICYT.

Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA); the observatory was made possible by the generous financial support of the W. M. Keck Foundation. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from the NSF. NTT data was obtained under the ESO Programme. Part of the analysis occurred at the Aspen Center for Physics. Some analysis was performed at the Woody Creek Community Center in Woody Creek, CO.

The Carnegie Institution for Science (carnegiescience.edu) is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.