Culprit for Enigmatic Supernova?

Astronomers have detected a star in pre-explosion images of the peculiar supernova 2012Z. The detection is the first discovery of a potential progenitor for the oddball class of stellar explosions called Type Iax.

Astronomers have potentially found the progenitor of a mysterious type of supernova. These supernovae, dubbed Type Iax, look a lot like their beloved Type Ia cousins — except they’re between 1% and 50% as bright and they spew ejecta out at lower velocities.

Type Ia supernovae happen (we think) when a white dwarf steals too much material from a companion star, exceeds its mass limit, and goes kablooey. Astronomers love these explosions because they all have the same intrinsic luminosity, so observers can use a Type Ia’s apparent brightness to gauge the distance not only to the supernova but to the galaxy that contains it, building a cosmic distance ladder.

The most popular explanation for Type Iax events is that they’re created the same way as Type Ia ones, except the explosion doesn’t completely tear the white dwarf into bits. Instead, the white dwarf ejects roughly half of its mass, leaving behind a hot, radioactive carbon-oxygen core.

To explore these oddball events, Curtis McCully (Rutgers) and colleagues took a closer look at SN 2012Z, a Type Iax discovered in the galaxy NGC 1309 by the Lick Observatory Supernova Search. They used archival observations from the Hubble Space Telescope from 2005, 2006, and 2010, as well as new observations they took in 2013, to study what existed before and after the supernova appeared.

The pre-supernova observations reveal a bright, bluish source the team calls S1, right where the supernova went off. Given its appearance, the astronomers don’t think it’s a type of star that was about to explode. So instead they argue that it’s the companion that fed a white dwarf’s thermonuclear outburst.

The detection is the first identification of a potential progenitor system for these strange events. “The findings of McCully et al. are very exciting,” says stellar explosion expert Markus Kromer (Stockholm University, Sweden). Once the supernova fades, astronomers will be able to tell whether S1 is still there. If it is, that means the accreting-white-dwarf scenario is probably correct. “Knowing which of these possibilities is realized in nature will be a great breakthrough in our understanding of Iax supernovae.” And it’ll have huge implications on our understanding of the standard Type Ia events, too, he adds.

A Muddled Helium Pudding

What kind of star S1 is remains unclear. The team argues that it looks like a helium star, a star that’s lost its outer hydrogen envelope due to interactions with its white dwarf companion. There’s no helium apparent in SN 2012Z’s spectra, but it’s hard to create these helium spectral lines, so if the helium is there it might be hiding. It might also have been so efficiently fused into carbon and oxygen that there’s not much left to make a spectral stink, adds coauthor Saurabh Jha (Rutgers) — one of many potential explanations.

But Kromer cautions that the helium-star explanation might not be the most probable. Theorists suspect that accreted helium layers are more unstable than hydrogen ones, and a wayward detonation might go off and trigger the white dwarf’s demise before it has a chance to exceed its mass limit. That “double-detonation” would also give rise to a supernova, but its properties wouldn’t match those of a Type Iax.

Study coauthor Ryan Foley (University of Illinois at Urbana-Champaign) agrees that the double-detonation scenario is typical. But an extremely high accretion rate could fuel stable helium fusion in the white dwarf’s outer layers. Or perhaps smaller helium flashes — like that behind the helium nova V445 Puppis, which looked like 2012Z — help stave off death until the dwarf grows too massive. They'll need more observational and theoretical work to answer this question.

The supernova should fade enough by late 2015 to reveal whether S1 endures. The white dwarf itself might be detectable, too, thanks to the puffing-up effect the energy of radioactive decay would have on the dwarf’s outer layers. In fact, the team also observed the faded Type Iax SN 2008ha and found what looks like a reddish star, but they can’t tell whether it’s the white dwarf’s companion or the remnant of the dwarf itself.

On the other hand, if S1 is not still shining in NGC 1309, that’ll suggest S1 itself exploded and the supernova was the death of a massive star — which would send astronomers back to the drawing board.

 

References:

Curtis McCully et al. “A luminous, blue progenitor system for the type Iax supernova 2012Z.” Nature. August 7, 2014

Ryan J. Foley at al. “Possible Detection of the Stellar Donor or Remnant for the Type Iax Supernova 2008ha.” Accepted to Astrophysical Journal.

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