The star that survived a supernova

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

So when a team of astronomers went to see the site of the strange 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, but the star was even brighter after the supernova than it had been before.

First author Curtis McCully, a postdoctoral researcher at UC Santa Barbara and the Las Cumbres Observatory, published these results in a paper by The Astrophysical Journal and presented them at a press conference at the 240th meeting of the American Astronomical Society. The puzzling results give us new insights into the origins of some of the most common, yet mysterious, explosions in the universe.

These thermonuclear supernovae, also called type Ia supernovae, are among the most important tools in astronomers’ toolkit for measuring cosmic distances. Beginning in 1998, observations of these explosions revealed that the universe was expanding at an ever-increasing rate. This is thought to be due to dark energywhose discovery won the Nobel Prize in Physics in 2011.

Although vitally important to astronomy, the origins of thermonuclear supernovae are poorly understood. Astronomers agree that it is the destruction of white dwarf stars – stars roughly the mass of the sun crammed into the size of the Earth. What makes the stars explode is unknown. One theory posits that the white dwarf steals matter from a companion star. When the white dwarf becomes too heavy, 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 referred to as an Iax-type supernova. They are the weaker, weaker cousins ​​of the more traditional Type Ia. Because they are less powerful and slower explosions, some scientists have speculated that they were failed Type Ia supernovae. The new observations confirm this hypothesis.

In 2012, supernova 2012Z was detected in nearby spiral galaxy NGC 1309, which had been studied extensively and captured in numerous Hubble images in the years leading up to 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. Analysis of this data in 2014 was successful – scientists were able to identify the star at the exact position of supernova 2012Z. It was the first time that the progenitor star 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 it might still be there, which means that the star we saw in the pre-explosion images was not the one that exploded. to see a brighter surviving star. . It was a real headache.”

McCully and the team believe the half-exploded star got brighter because it swelled to a much larger state. The supernova wasn’t strong enough to wash away all the material, 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 they are in diameter.

“This surviving star is kind of like Obi-Wan Kenobi returning as a Force Ghost in Star Wars,” said co-author Andy Howell, assistant professor at UC Santa Barbara and principal investigator at Las Cumbres Observatory. . “Nature tried to knock this star down, but it came back stronger than we could have imagined. It’s still the same star, but in a different form. It transcended death.”

For decades, scientists thought that Type Ia supernovae exploded when a white dwarf star reached a certain size limit, called the Chandrasekhar limit, or about 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 didn’t know if the stars ever approached the Chandrasekhar boundary before exploding. The study authors now believe that this growth to the ultimate limit is exactly what happened to SN 2012Z.

“The implications for Type Ia supernovae are profound,” McCully says. “We constated that supernovae at least can grow to the limit and explode. Still, the explosions are weak, at least some of the time. Now we need to understand what makes a supernova fail and become Type Iax, and what makes a supernova succeed as Type Ia.”