There are different types of supernova explosions. Type Ia supernovaealso called 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 may be a red giant. Type Ia supernovae signal the complete destruction of a white dwarf, leaving nothing behind. So when astronomers went to see the site of 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, but the star was even brighter after the supernova than it had been before.
Color images of NGC 1309 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 Iax-type supernova. They are the weaker, weaker cousins of the more traditional Type Ia.
Because they are less powerful and slower explosions, some astronomers have speculated that they were failed Type Ia supernovae. The new observations confirm this hypothesis.
SN 2012Z was detected in January 2012 at NGC 1309a spiral galaxy located about 110 million light-years away in the constellation Eridanus, which had been extensively studied and capture in many Hubble images in the years leading up to SN 2012Z.
The Hubble images were taken in 2013 in a concerted effort to identify which star in 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. It was the first time that the progenitor star of a white dwarf supernova was identified.
“We expected to see one of two things when we got the most recent Hubble data,” said Dr. Curtis McCully, postdoctoral researcher at Las Cumbres Observatory and the University of California, Santa Barbara.
“Either the star would have completely disappeared, or it might still be there, which means that the star we saw in the images before the explosion was not the one that exploded. No one expected to see a brighter surviving star. It was a real headache.”
Dr McCully and his colleagues believe the half-exploded star got brighter because it swelled to a much larger state.
SN 2012Z wasn’t strong enough to blow out all the material, so some fell back into what’s called a bonded 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 supernovae exploded when a white dwarf reached a certain size limit, called the Chandrasekhar boundaryabout 1.4 times the mass of the sun.
This model has fallen somewhat out of favor in recent years, as many supernovae have turned out to be less massive than this, and new theoretical ideas have indicated that there are other things that cause them to explode.
Astronomers weren’t sure if the stars ever approached the Chandrasekhar boundary before exploding.
The study authors now believe that this growth at the ultimate limit is exactly what happened to SN 2012Z.
“The implications for Type Ia supernovae are profound,” said Dr McCully.
“We found that supernovae, at least, can grow to the limit and explode. Still, the explosions are weak, at least occasionally.
“Now we need to understand what makes a supernova fail and become Type Iax, and what makes a supernova succeed as Type Ia.”
The results appear in the Astrophysical Journal.
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Curtis McCully et al. 2022. Still brighter than the pre-explosion, SN 2012Z has not disappeared: comparison of Hubble Space Telescope observations a decade apart. ApJ 925, 138; doi: 10.3847/1538-4357/ac3bbd
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