The star that survived a supernova

A supernova is the catastrophic explosion of a star. Thermonuclear supernovas in particular signal the complete destruction of a white dwarf star, leaving nothing behind. At least, that’s what models and observations suggested.

So when a team of astronomers went to look at the site of the peculiar thermonuclear? supernova SN 2012Z with the Hubble Space Telescope, they were shocked to find that the star had survived the explosion. Not only had it survived, the star was even brighter after the supernova than before.

First author Curtis McCully, a postdoctoral researcher at UC Santa Barbara and Las Cumbres Observatory, published these findings in an article in The Astrophysical Journal and presented them at a press conference at the 240th Meeting of the American Astronomical Society. The puzzling results provide us with 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’ toolkits for measuring cosmic distances. From 1998, observations of these explosions revealed that the universe was expanding at an accelerating rate. This is thought to be due to: dark energywhose discovery won the Nobel Prize in Physics in 2011.

Though vital to astronomy, the origins of thermonuclear supernovae are poorly understood. Astronomers agree that they are the destruction of white dwarf stars — stars that are roughly the mass of the Sun, packed into the size of Earth. What causes the stars to explode is unknown. One theory holds that the white dwarf steals matter from a companion star† When the white dwarf becomes too massive, thermonuclear reactions ignite in the core and lead to a runaway explosion that destroys the star.

SN 2012Z was a strange type of thermonuclear explosion, sometimes 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 scientists have theorized that they are failed Type Ia supernovas. The new observations confirm this hypothesis.

In 2012, the 2012Z supernova was detected in nearby spiral galaxy NGC 1309, which had been thoroughly studied and captured in many Hubble images leading up to 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. Analysis of this data in 2014 was successful: Scientists were able to identify the star at the exact position of the 2012Z supernova. This was the first time the progenitor of a white dwarf supernova had been identified.

“We expected to see one of two things when we got the most recent Hubble data,” McCully said. “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. No one expected to see a surviving star that was brighter. That was a real puzzle.”

McCully and the team think the half-exploded star brightened as it blew up to a much larger state. The supernova 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 dwarf stars, the less mass they have, the larger their diameter.

“This surviving star is a bit like Obi-Wan Kenobi returning as a power ghost in Star Wars,” said study co-author Andy Howell, an adjunct professor at UC Santa Barbara and senior staff scientist at Las Cumbres Observatory. “Nature tried to bring this star down, but it came back more powerful than we could have imagined. It’s still the same star, but back in a different shape. It transcends death.”

For decades, scientists thought that Type Ia supernovas explode when a white dwarf star reaches a certain limit, the Chandrasekhar limit, about 1.4 times the mass of the Sun. That model has fallen somewhat out of favor in recent years, as many supernovas 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,” McCully says. “We have found that supernovas can grow and explode in any case. 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.”