The star that survived a supernova

A supernova is the catastrophic explosion of a star. fusion supernova, especially signaling the complete annihilation of a white dwarf star, leaving nothing behind. At least that’s what the models and observations have suggested.

Galaxy NGC 1309. Image providers: NASA, ESA, Hubble Heritage Group (STSCI/AURA), A. Riess (JHU/STSCI).

So, when a team of astronomers came to examine the site of the exceptional thermonuclear supernova SN 2012Z using the Hubble Space Telescope, they were shocked to discover that the star had survived. explosion. It doesn’t just exist – the star is even brighter after the supernova than before. First author Curtis McCullya postdoctoral researcher at UC Santa Barbara and the Las Cumbres Observatory, published these findings in a paper in Astrophysics Magazine and presented them in a press conference at the 240th meeting of the American Astronomical Society. The puzzling results give us new information about the origins of some of the most common, yet mysterious, explosions in the universe.

These thermonuclear supernovas, also known as Type Ia supernovas, are some of the most important tools in astronomers’ toolkit for measuring cosmic distances. Beginning in 1998, observations of these outbursts showed that the universe was expanding at an increasingly rapid rate. This is attributed to dark energy, the discovery of which won the 2011 Nobel Prize in Physics.

While they are extremely important to astronomy, the origins of thermonuclear supernovae are still poorly understood. Astronomers agree that they are the annihilation of white dwarfs – stars about the same mass as the sun and about the same size as Earth. The cause of stars exploding is still unknown. One theory is that the white dwarf steals matter from a companion star. When a white dwarf becomes too massive, fusion reactions ignite in the core and lead to an explosion that destroys the star.

SN 2012Z is ​​a strange type of thermonuclear explosion, sometimes referred to as a Type Iax supernova. They are weaker, dimmer cousins ​​of the more traditional Type Ia. Because they are less powerful and the explosion is slower, some scientists have hypothesized that they are Type Ia supernovas that have failed. New observations confirm this hypothesis.

In 2012, supernova 2012Z was detected in the nearby spiral galaxy NGC 1309, which has been intensively studied and captured in many Hubble images in the years leading up to 2012Z. The Hubble image was taken in 2013 in a concerted effort to determine which of the older images corresponded to the star that exploded. The analysis of this data in 2014 was successful – scientists were able to identify the star at the exact location of supernova 2012Z. This is the first time that the precursor star of a white dwarf supernova has been identified.

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

“We were expecting to see one of two things when we got the most recent Hubble data,” McCully said. “Maybe the star has disappeared completely, or maybe it is still there, which means that the star we see in the pre-explosion images is not the star that exploded. No one expected to see a brighter remnant star. It’s a real puzzle. “

McCully and the team think that half of the star explodes brighter because it swells to a much larger state. The supernova wasn’t powerful enough to blow away all the matter, so some of it fell back into what’s known as bound ruins. Over time, they expect the star to slowly return to its original state, only with less and more mass. Paradoxically, for white dwarfs, the smaller their mass, the larger their diameter.

“This survivor star a bit like Obi-Wan Kenobi returns as a force specter in Star Wars,” said co-author Andy Howell, assistant professor at UC Santa Barbara and senior staff scientist at the Las Cumbres Observatory. “Nature tried to throw this star down, but it came back stronger than we could have imagined. It’s still an old star, but back in a different form. It has overcome death”.

For decades, scientists thought that Type Ia supernovae explode when a white dwarf star reaches a certain limit in size, known as the Chandrasekhar limit, about 1.4 times the mass of Sun. That model has fallen out of favor in the last few years, as many supernovae have been found to have masses less than this, and new theoretical ideas have shown that there are other things that cause them to explode. . Astronomers aren’t sure if stars ever come close to the Chandrasekhar limit before exploding. The study authors now suggest that this ultimate growth is exactly what happened with SN 2012Z.

“The implications for Type Ia supernovae are profound,” says McCully. “We found that the supernova can at least grow to its limit and explode. However, the explosions are very weak, at least for some time. We now need to understand what causes a supernova to fail and become a type Iax, and what causes a supernova to succeed as a type Ia.”

Source: UC Santa Barbara

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