New Kind of “Runt” Supernovae Could be Lurking Unseen

This artist's conception shows the suspected progenitor of a new kind of supernova called Type Iax. Material from a hot, blue helium star at right is funneling toward a carbon/oxygen white dwarf star at left, which is embedded in an accretion disk. In many cases the white dwarf survives the subsequent explosion. Credit: Christine Pulliam (CfA)

Imagine this “Death from the Skies” scenario; a tiny supernova lurks unseen near our Sun. Astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) announced the discovery of just such an object today and while it is not nearby, this new kind of supernova is so faint it has been hiding in the shadows.

Until now, supernovae have come in two main versions. In one scenario, a huge star, 10 to 100 times more massive as our Sun, collapses causing a colossal stellar explosion. Another scenario, known as Type Ia supernovae, occurs when material from a parent star streams onto the surface of a white dwarf. Over time, so much material falls onto the white dwarf that it raises the core temperature igniting carbon and causing a runaway fusion reaction. This event completely disrupts the white dwarf and results in a colossal stellar explosion.

Now astronomers have found a third type that is fainter and less energetic than a Type Ia. Called a Type Iax supernova, it is “essentially a mini supernova,” says lead author of the study Ryan Foley, Clay Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). “It’s the runt of the supernova litter.”

Being only about one-hundredth as bright as their supernova siblings, Foley calculates that Type Iax supernovae are about as third as common as Type Ia supernovae. The researchers also did not find them in elliptical galaxies, filled with older stars, suggesting that Type Iax supernovae come from young star systems.

So far, Foley and his team identified 25 examples of this new type of supernova. Based on observations, the team found that the new Type Iax supernovae come from binary star systems containing a white dwarf and a companion star that has burned all of its hydrogen, leaving an outer layer that is helium rich.

In a press release, Foley says they are not sure what triggers the Type Iax supernova. One explanation involves the ignition of the outer helium layer from the companion star. The resulting shockwave slams into the white dwarf and disrupts it, causing the explosion. Alternately, the white dwarf might ignite first due to the overlying helium shell it has collected from the companion star.

“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,” says Foley. “The star will be battered and bruised but it might live to see another day.”

Supernovae explosions release so much energy as heat and light that they outshine entire galaxies for brief periods of time. The extremely hot conditions naturally create new heavier elements, such as gold, lead, nickel, zinc and copper. The explosion enriches the surrounding area leaving material for new stars to form.

“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.”

This research has been accepted for publication in The Astrophysical Journal and is available online.