There are different types of supernova explosions. Type Ia supernovae, also known as thermonuclear supernovae, are found in binary star systems. To activate 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 it may be a red giant star. Type Ia supernovae indicate the complete destruction of a white dwarf, leaving nothing behind. Thus, when astronomers went to look at the site of the 2012Z supernova (SN) with NASA / ESA’s Hubble Space Telescope, they were surprised to discover that the star had survived the explosion. Not only had it survived, but the star was even brighter after the supernova than 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 a Iax-type supernova. They are the weakest and weakest cousins of the more traditional type Ia.
Because they are less powerful and slower explosions, some astronomers have theorized that they are failed Type I supernovae. The new observations confirm this hypothesis.
SN 2012Z was detected in January 2012 at NGC 1309, a spiral galaxy located about 110 million light-years away in the constellation Eridanus, which had been studied in depth and captured in many Hubble images. during the years prior to SN 2012Z.
Hubble images were taken in 2013 in a concerted effort to identify which star in the oldest images corresponded to the star it had exploded.
The 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 the parent star of a white dwarf supernova was identified.
“We were expecting to see one of two things when we got the latest data from Hubble,” said Dr. Curtis McCully, a postdoctoral researcher at Las Cumbres Observatory and the University of California, Santa Barbara.
“Either the star would have disappeared completely, or maybe it would still have been there, that is, the star we saw in the pre-explosion images was not the one that exploded. No one expected to see a surviving star that was brighter. It was a real puzzle. “
Dr. McCully and his colleagues think the half-exploded star became brighter because it swelled to a much larger state.
The 2012Z SN was not strong enough to blow up all the material, so one part fell back into what is called a tied 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 explode when a white dwarf reaches a certain size limit, called the Chandrasekhar limit, about 1.4 times the mass of the sun.
This model has fallen a bit into disgrace in recent years, as many supernovae have been found to be less massive than this one, and new theoretical ideas have indicated that there are other things that make them explode.
Astronomers were unsure whether the stars had approached the Chandrasekhar limit before exploding.
The study authors now think that this growth to the final limit is exactly what happened with SN 2012Z.
“The implications for type Ia supernovae are profound,” Dr. McCully said.
“We’ve found that supernovae can at least grow to the limit and explode. However, explosions are weak, at least sometimes.”
“Now we need to understand what makes a supernova fail and become an Iax type, and what makes a supernova successful as type Ia.”
The results appear in the Astrophysical Journal.
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Curtis McCully et al. 2022. Even brighter than the pre-explosion, SN 2012Z did not disappear: comparing the observations of the Hubble Space Telescope with a decade of difference. ApJ 925, 138; doi: 10.3847 / 1538-4357 / ac3bbd