Galaxy NGC 1309

Strange star survived the thermonuclear supernova

A supernova is the catastrophic explosion of a star. A thermonuclear supernova explosion of a white dwarf would be the last event in the life of its ancestor star.

Known as Type Ia supernovas, these thermonuclear supernovas are some of the most important tools in astronomers’ toolkits for measuring cosmic distances. The explosion is thought to completely dislodge the star for a normal type Ia supernova (SN Ia), the kind used as a standardizable candle for cosmology.

A team of astronomers has used the Hubble Space Telescope to explore the location of the peculiar thermonuclear supernova SN 2012Z. In a shocking revelation, they found that the star had survived the explosion.

Not only had it survived, but the star was even brighter after the supernova than before.

First author Curtis McCully, a postdoctoral researcher at the University of CaliforniaSanta Barbara and Las Cumbres Observatory, presented these findings at a press conference at the 240th meeting of the American Astronomical Society and published them in an article in The Astrophysical Journal

The mind-boggling results provide new information about the origins of some of the most common yet mysterious explosions in the universe. It also made clear that there is a greater variety of white dwarf supernovae than just the typical SN Ia.

The origin of thermonuclear supernovae is poorly understood, despite their vital importance to astronomy. Astronomers agree they are the destruction of the white dwarf stars(stars about the mass of the sun packed into the size of the earth).

What causes the destruction of a white dwarf?

The reason is unknown. One theory suggests that the white dwarf steals matter from a companion star. When the white dwarf becomes too massive, thermonuclear reactions ignite in the core, leading to a runaway explosion that destroys the star.

What could be the reason behind the star’s survival and brightness?

Type Iax supernovas (SNe Iax) are the most populous class of peculiar white dwarf supernovae. Based on the prototype SN 2002cx, these are sub-luminous, low-velocity explosions compared to normal SNe Ia. In particular, there is mounting evidence that SNe Iax may not completely destroy the star, but instead leave a bound remnant, unlike the complete disruption expected in SNe Ia, the study mentions.

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, because white dwarf stars, the smaller their masses, the larger they are in 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.”

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

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 are 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 believe that this growth to the extreme is exactly what happened to SN 2012Z.

“The implications for Type Ia supernovas are profound,” McCully says. “We’ve found that supernovas can grow and explode at the very least. Still, the explosions are faint, at least some of the time. Now we need to understand what a supernova failing and becoming a Type Iax, and what makes someone successful as a Type Ia.”

Futuristic view:

“We encourage others to use our observations as constraints for their simulations to better understand the physical mechanisms that produce SNe Iax.” Study mentions.

Magazine reference

  1. Curtis McCully, Saurabh W. Jha, Richard A. Scalzo, D. Andrew Howell, Ryan J. Foley, Yaotian Zeng, Zheng-Wei Liu, Griffin Hosseinzadeh, Lars Bildsten, Adam G. Riess, Robert P. Kirshner, GH Marion and Yssavo Camacho-Neves. Even brighter than pre-explosion, SN 2012Z didn’t disappear: Hubble Space Telescope observations compare ten years apart. The Astrophysical JournalVolume 925, Number 2. DOI: 10.3847/1538-4357/ac3bbd

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