There are different types of supernova explosions. Type Ia supernovaealso known as thermonuclear supernovas, to prevent in double 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, such as our sun, or may be a red giant star. Type Ia supernovae signal the complete destruction of a white dwarf, leaving nothing behind. So when astronomers started looking at the location 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 before.
SN 2012Z is a type of thermonuclear explosion called a Type Iax supernova† They are the weaker, 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 supernovas. The new observations confirm this hypothesis.
SN 2012Z was detected in January 2012 in NGC 1309a spiral galaxy about 110 million light-years away in the constellation Eridanus, which had been thoroughly studied and captured in many Hubble images leading up to SN 2012Z.
Hubble images were taken in 2013 in a collaborative effort to identify which star in the older images matched the star that had exploded.
An analysis of this data in 2014 was successful: Scientists were able to identify the star at the exact position of SN 2012Z. This was the first time the ancestor of a white dwarf supernova was identified†
“We expected to see one of two things when we got the most recent Hubble data,” says Dr. Curtis McCully, a postdoctoral researcher at Las Cumbres Observatory and the University of California, Santa Barbara.
“Either the star would have completely disappeared, or maybe it would still be there, meaning the star we saw in the pre-explosion images wasn’t the one that blew up. Nobody expected to see a surviving star that was brighter. That was a real puzzle.”
dr. McCully and colleagues think the half-exploded star brightened as it blew up to a much larger state.
SN 2012Z wasn’t strong enough to blow all the material away, so some of it fell back into what’s called a bound remnant.
Over time, they expect the star to slowly return to its original 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 supernovas explode when a white dwarf reaches a certain limit, called the Chandrasekhar Limitabout 1.4 times the mass of the sun.
That model has fallen somewhat out of favor in recent years, as many supernovae have been found to be less massive than this one, and new theoretical ideas have shown that there are other things that cause them to explode.
Astronomers weren’t sure if stars ever got close to the Chandrasekhar limit before exploding.
The study authors now think that this growth to the extreme is exactly what happened to SN 2012Z.
“The implications for Type Ia supernovas are profound,” said Dr. McCully.
“We’ve found that supernovae can grow and explode at the very least. Still, the explosions are faint, at least some of the time.”
“Now we need to understand why a supernova fails and becomes a Type Iax, and what makes someone successful as a Type Ia.”
The Results appear in the Astrophysical Journal†
Curtis McCully et al† 2022. Still brighter than pre-explosion, SN 2012Z didn’t disappear: Hubble Space Telescope observations compared a decade apart. ApJ 925, 138; doi: 10.3847/1538-4357/ac3bbd
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