The star that survived a supernova

       Galaxy NGC 1309. Credit: NASA, ESA, The Hubble Heritage Team (STSCI/AURA), and A. Riess (JHU/STSCI)

The catastrophic stellar explosion known as a supernova. Particularly thermonuclear supernovae signify a white dwarf star's total death, leaving nothing behind. Models and observations seemed to support this, at least.

So it came as a surprise to scientists when they used the Hubble Space Telescope to examine the location of the strange thermonuclear supernova SN 2012Z that the star had survived the explosion. The star had not only survived, but after the explosion it was considerably brighter than before.

These findings were presented at a press conference at the 240th meeting of the American Astronomical Society by the study's first author, Curtis McCully, a postdoctoral researcher at UC Santa Barbara and Las Cumbres Observatory who also published them in an article in The Astrophysical Journal. The perplexing findings reveal fresh details regarding the causes of some of the frequent yet enigmatic explosions in the cosmos.

These Type Ia thermonuclear supernovae are among the most crucial instruments in astronomers' toolkits for determining cosmic distances. The universe has been expanding at an ever-accelerating rate since 1998, according to measurements of these explosions. The 2011 Nobel Prize in Physics was awarded for the discovery of dark energy, which is assumed to be the cause of this.

The origins of thermonuclear supernovae are poorly understood, despite the fact that they are crucial to astronomy. They are believed to be the demise of white dwarf stars, which are stars the size of the Earth with a mass similar to that of the sun. It is uncertain what triggers stellar explosions. According to one theory, the white dwarf takes materials from a nearby star. The star is obliterated by a runaway explosion caused by thermonuclear processes that start when the white dwarf becomes too massive.

A Type Iax supernova, or SN 2012Z, was a peculiar kind of thermonuclear explosion. They are the weaker, less vibrant relatives of the more conventional Type Ia. Some scientists have proposed that they are failed Type Ia supernovae because of their weaker and delayed explosions. The latest findings support this theory.

Left: Color image of Galaxy NGC 1309 before Supernova 2012Z. Right: Clockwise from top right: the position of the supernova pre-explosion; SN~2012Z during the 2013 visit; the difference between the pre-explosion images and the 2016 observations; the location of SN~2012Z in the latest observations in 2016. Credit: McCully et al

The nearby spiral galaxy NGC 1309, which had undergone extensive research and been photographed in numerous Hubble photos in the years before to 2012Z, was where the supernova 2012Z was discovered in 2012. In a deliberate effort to determine which star in the earlier photographs matched the star that had exploded, Hubble images were taken in 2013. Scientists successfully identified the star at the precise location of the supernova 2012Z in 2014 after analyzing this data. It had never before been possible to pinpoint the star that gave rise to a white dwarf supernova. 

"We were expecting to see one of two things when we got the most recent Hubble data," McCully said. "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."

McCully and the colleagues believe that the half-exploded star's increased brightness is a result of its expansion into a much larger state. Some of the debris drifted back into what is known as a bound remnant since the supernova wasn't powerful enough to completely blow away all of it. They anticipate that the star will gradually revert to its original state over time, but with a smaller and greater mass. Contrary to popular belief, white dwarf stars are paradoxically greater in diameter as their mass decreases. 

"This star surviving is a little like Obi-Wan Kenobi coming back as a force ghost in Star Wars," said co-author Andy Howell, adjunct professor at UC Santa Barbara and senior staff scientist at Las Cumbres Observatory. "Nature tried to strike this star down, but it came back more powerful than we could have imagined. It is still the same star, but back in a different form. It transcended death."

For many years, it was believed by scientists that white dwarf stars could not grow any larger than the Chandrasekhar limit, which is equivalent to 1.4 times the mass of the sun. Since several supernovae have been discovered to be less massive than this one and new theoretical concepts have suggested that there are alternative causes for them to burst, this hypothesis has partially lost favor in recent years. If stars ever came close to the Chandrasekhar limit before exploding was a question that puzzled astronomers. The authors of the study now believe that SN 2012Z experienced this growth to the absolute limit. 

"The implications for Type Ia supernovae are profound," says McCully. "We've found that  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."

More information: Curtis McCully et al, Still Brighter than Pre-explosion, SN 2012Z Did Not Disappear: Comparing Hubble Space Telescope Observations a Decade Apart, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac3bbd

Provided by University of California - Santa Barbara

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