Friday, August 12, 2022

Strange star survived the thermonuclear supernova

It actually became even brighter after a supernova.

A supernova is the cataclysmic explosion of a star. A thermonuclear supernova explosion of a white dwarf is supposed to be the terminal event in the life of its progenitor star.

These thermonuclear supernovae, known as Type Ia supernovae, are some of the most important tools in astronomers’ toolkits for measuring cosmic distances. For a normal type Ia supernova (SN Ia), which is the kind used as a standardizable candle for cosmology, the explosion is thought to completely unbind the star.

A team of astronomers examined the site of the peculiar thermonuclear supernova SN 2012Z with the Hubble Space Telescope. In a shocking revelation, they discovered 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 the University of California, Santa 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 perplexing results provide new information about the origins of some of the universe’s most common yet mysterious explosions. It also clarified that there is a broader variety of white dwarf supernovae than just typical SN Ia.

The origins of thermonuclear supernovae are poorly understood, despite their vital importance to astronomy. Astronomers agree that they are the destruction of white dwarf stars(stars roughly 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 gets too heavy, thermonuclear reactions ignite in the core, leading to a runaway explosion that destroys the star.

What could be the reason behind the survival and brightness of the star?

“Type Iax supernovae (SNe Iax) comprise the most populated 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 fully destroy the star, but instead may leave behind a bound remnant, in contrast to the complete disruption expected in SNe Ia.” the Study mentions.

McCully and the team think the half-exploded star got brighter because it puffed up to a much bigger state. The supernova wasn’t strong enough to blow away all the material, so some of it fell back into what is called a bound remnant. Over time, they expect the star to slowly return to its initial state, only less massive and larger. Paradoxically, for white dwarf stars, the smaller their mass, the larger they are in diameter.

“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.”

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.
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. Photo Credit: MCCULLY ET AL.

For decades scientists thought that Type Ia supernovae explode when a white dwarf star reaches a certain limit in size, called the Chandrasekhar limit, about 1.4 times the mass of the sun. That model has fallen somewhat out of favor in the last few years, as many supernovae are less massive than this, and new theoretical ideas have indicated that there are other things causing them to explode. Astronomers were unsure if stars ever got near the Chandrasekhar limit before exploding. The study authors now think that this growth to the ultimate limit is exactly what happened to SN 2012Z.

“The implications for Type Ia supernovae are profound,” says McCully. “We’ve found that supernovae 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.”

Futuristic view:

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

Journal 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, G. H. Marion and Yssavo Camacho-Neves. Still Brighter than Pre-explosion, SN 2012Z Did Not Disappear: Comparing Hubble Space Telescope Observations a Decade Apart. The Astrophysical Journal, Volume 925, Number 2. DOI: 10.3847/1538-4357/ac3bbd
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