(Sen) - Astronomers have discovered a new type of supernova.
Until now there have been two classes of supernovas known, both are extremely luminous events that cause a burst of radiation that can outshine a whole galaxy. A core-collapse supernova is the explosion of a star about 10 to 100 times as massive as the Sun. A Type Ia, or thermal runaway supernova, occurs in binary systems, with one star being a tiny white dwarf, when material from the parent star streams onto its surface resulting in a runaway fusion reaction and complete destruction of the white dwarf.
The new class of supernova, called a Type Iax, is fainter and less energetic than Type Ia. Although both varieties 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."
Foley and his colleagues identified 25 examples of the new type of supernova. None of them appeared in elliptical galaxies, which are filled with old stars, which suggests that Type Iax supernovas come from young star systems. The team conclude 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 dominated by helium. 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 star will be battered and bruised, but it might live to see another day," says Foley.
It has been calculated that Iax supernovas are about a third as common as Type Ia supernovas. The reason so few have been detected is that the faintest are only one-hundredth as bright as a Type Ia.
"Type Iax supernovas aren't rare, they're just faint," explains Foley. "For more than a thousand years, humans have been observing supernovas. This whole time, this new class has been hiding in the shadows."
The CfA is a partner in the Large Synoptic Survey Telescope, which they hope could discover thousands of Type Iax supernovas over its lifetime. Their research has been accepted for publication in The Astrophysical Journal.