Top Story: August 7, 2014

Explosions, Bullies, and Zombies

Hubble Space Telescope images of the spiral galaxy NGC 1309, located 108 million light-years away, were taken before and after the appearance of Supernova 2012Z in the outskirts of the galaxy. The inset panel from 2013 shows the supernova, while the data from 2005 and 2006 show the progenitor system for the supernova, thought to be a binary system containing a helium star transferring material to a white dwarf that exploded.

By Molly Michelson

This astronomy tale has it all: explosions, bullies, and zombies! Sounds like a summer blockbuster, right? Maybe it is…

Researchers, publishing this week in Nature, may have found the elusive progenitor, or stellar source, of a weak supernova. It’s kind of a big deal.

The supernova in question is SN 2012Z, which took place in the galaxy NGC 1309, about 110 million light years away from Earth. As you might guess from its name, SN 2012Z was discovered in 2012. Luckily, the Hubble Space Telescope had observed NGC 1309 for several years prior the supernova outburst, which allowed scientists to compare before-and-after images.

While examining the images taken years before the stellar explosion, astronomers identified a binary system with a blue companion star feeding energy to a white dwarf, a process that ignited a nuclear reaction and released this weak supernova blast. This kind of supernova, Type Iax, is less common than its brighter cousin, Type Ia. Astronomers have identified more than 30 of these mini-supernovas that may leave behind a surviving white dwarf.

“Astronomers have been searching for decades for the star systems that produce Type Ia supernova explosions,” says scientist and study co-author Saurabh Jha of Rutgers University. “Type Ia’s are important because they’re used to measure vast cosmic distances and the expansion of the universe.” It turns out that, although Type Ia’s are very bright, their progenitors are quite dim—and difficult to find!

“I was very surprised to see anything at the location of the supernova,” says grad student and study lead author Curtis McCully, also of Rutgers, of the potential supernova source. “We expected  the progenitor system would be too faint to see, like in previous searches for normal Type Ia supernova progenitors. It is exciting when nature surprises us.”

One possible explanation for the unusual nature of SN 2012Z is that a game of seesaw ensued between the bigger and smaller of the star pair. The more massive star evolved rapidly. As it expanded, it dumped its hydrogen and helium onto the smaller star and quickly became a white dwarf. The smaller star bulked up, grew into a big bully star, and engulfed the white dwarf. The outer layers of this combined star were ejected, leaving behind the white dwarf and the helium core of the companion star. The white dwarf ultimately got its revenge on the bully, siphoning matter from the companion star and eventually exploding as a mini-supernova, leaving behind a surviving zombie star.

The team plans to use Hubble again in 2015 to observe the region, giving time for the supernova’s light to dim enough to reveal the possible zombie star and helium companion to confirm their hypothesis.

So perhaps this blockbuster will have a happy ending, for the astronomers if not for the stars.

Image: NASA and ESA, Curtis McCully and Saurabh W Jha (Rutgers), Ryan J Foley (Illinois)

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