Hubble Finds Half-Exploded Star in NGC 1309

by johnsmith

There are different types of supernova explosions. Type Ia supernovae, also known as thermonuclear supernovae, occur in binary star systems. To trigger a type Ia supernova, one of the two stars must be a white dwarf. The other star is often a low-mass star, like our Sun, or can be a red giant star. Type Ia supernovae signal the complete destruction of a white dwarf, leaving nothing behind. So when astronomers went to look at the site of the supernova (SN) 2012Z with the NASA/ESA Hubble Space Telescope, they were shocked to discover that the star had survived the explosion. Not only had it survived — the star was even brighter after the supernova than it had been before.

Color images of NGC 1309 both before and after SN 2012Z. Image credit: NASA / ESA / Hubble / McCully et al., doi: 10.3847/1538-4357/ac3bbd.

Color images of NGC 1309 both before and after SN 2012Z. Image credit: NASA / ESA / Hubble / McCully et al., doi: 10.3847/1538-4357/ac3bbd.

SN 2012Z is a type of thermonuclear explosion called a Type Iax supernova. They are the dimmer, weaker cousins of the more traditional Type Ia.

Because they are less powerful and slower explosions, some astronomers have theorized that they are failed Type Ia supernovae. The new observations confirm this hypothesis.

SN 2012Z was detected in January 2012 in NGC 1309, a spiral galaxy located about 110 million light-years away in the constellation of Eridanus, which had been studied in depth and captured in many Hubble images over the years leading up to SN 2012Z.

Hubble images were taken in 2013 in a concerted effort to identify which star in the older images corresponded to the star that had exploded.

An analysis of these data in 2014 was successful — scientists were able to identify the star at the exact position of SN 2012Z. This was the first time that the progenitor star of a white dwarf supernova had been identified.

“We were expecting to see one of two things when we got the most recent Hubble data,” said Dr. Curtis McCully, a postdoctoral researcher at Las Cumbres Observatory and the University of California, Santa Barbara.

“Either the star would have completely gone away, or maybe it would have still been there, meaning the star we saw in the pre-explosion images wasn’t the one that blew up. Nobody was expecting to see a surviving star that was brighter. That was a real puzzle.”

Dr. McCully and colleagues think that the half-exploded star got brighter because it puffed up to a much bigger state.

SN 2012Z wasn’t strong enough to blow away all the material, so some of it fell back into what is called a bound remnant.

Over time, they expect the star to slowly return to its initial state, only less massive and larger. Paradoxically, for white dwarfs, the less mass they have, the larger they are in diameter.

For decades, astronomers thought that Type Ia supernovae explode when a white dwarf reaches a certain limit in size, called the Chandrasekhar limit, about 1.4 times the mass of the sun.

That model has fallen somewhat out of favor in the past few years, as many supernovae have been found to be less massive than this, and new theoretical ideas have indicated that there are other things causing them to explode.

Astronomers were not sure if stars ever got near the Chandrasekhar limit before exploding.

The study authors now think that this growth to the ultimate limit is exactly what happened to SN 2012Z.

“The implications for Type Ia supernovae are profound,” Dr. McCully said.

“We’ve found that supernovae at least can grow to the limit and explode. Yet the explosions are weak, at least some of the time.”

“Now we need to understand what makes a supernova fail and become a Type Iax, and what makes one successful as a Type Ia.”

The results appear in the Astrophysical Journal.


Curtis McCully et al. 2022. Still Brighter than Pre-explosion, SN 2012Z Did Not Disappear: Comparing Hubble Space Telescope Observations a Decade Apart. ApJ 925, 138; doi: 10.3847/1538-4357/ac3bbd

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