The Hubble Space Telescope captured this spectacular spiral galaxy, listed as NGC 1309. Credits: NASA, ESA, The Hubble Heritage Team, (STScI / AURA) and A. Riess (STScI)
A tenacious star validates a revised supernova model
A supernova is the cataclysmic explosion of a star. Thermonuclear supernovae, in particular, indicate the complete destruction of a white dwarf star, leaving nothing behind. At least that is what astrophysics models and observations suggested.
Thus, when a team of astronomers went to examine the site of the peculiar thermonuclear supernova SN 2012Z with the Hubble Space Telescope, they were surprised to discover 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 California, Santa Barbara and the 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 puzzling results provide new information about the origins of some of the most common, but mysterious, explosions in the universe.
These thermonuclear supernovae, known as type Ia supernovae, are some of the most important tools in astronomers’ toolboxes for measuring cosmic distances. From 1998 onwards, observations of these explosions revealed that the universe has been expanding at an ever-accelerating pace. This is believed to be due to dark energy, the discovery of which won the Nobel Prize in Physics in 2011.
Left: Color image of the NGC 1309 galaxy before Supernova 2012Z. Right: Clockwise from top to right: the pre-explosion position of the supernova; SN ~ 2012Z during the 2013 visit; the difference between pre-explosion images and 2016 observations; the location of SN ~ 2012Z in the latest observations from 2016. Credit: McCully et al.
Although of vital importance to astronomy, the origins of thermonuclear supernovae are little known. Astronomers agree that they are the destruction of white dwarf stars: stars roughly the mass of the sun packed to the size of the Earth. It is unknown what causes the stars to explode. One theory is that the white dwarf steals matter from an accompanying star. When the white dwarf becomes too heavy, thermonuclear reactions ignite in the nucleus and cause an explosion that destroys the star.
Curtis McCully. Credit: UCSB
SN 2012Z was a strange type of thermonuclear explosion, sometimes called a Iax-type supernova. They are the weakest and weakest cousins of the more traditional type Ia. Because they are less powerful and slower explosions, some scientists have theorized that they are failed Type I supernovae. The new observations confirm this hypothesis.
In 2012, the 2012Z supernova was detected in the nearby spiral galaxy NGC 1309, which had been studied in depth and captured in many Hubble images in the years before 2012Z. Hubble images were taken in 2013 in a concerted effort to identify which star in the oldest images corresponded to the star it had exploded. The analysis of these 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 parent star of a white dwarf supernova was identified.
“We were expecting to see one of two things when we had the latest Hubble data,” McCully said. “Either the star would have disappeared completely, or maybe it would still have been there, that is, the star we saw in the pre-explosion images was not the one that exploded. No one expected to see a surviving star that was brighter. It was a real puzzle. “
McCully and the team think the half-exploded star became brighter because it swelled to a much larger state. The supernova was not strong enough to blow up all the material, so a part fell back into what is called a bounded debris. Over time, they expect the star to slowly return to its original state, only less massive and larger. Paradoxically, for white dwarfs, the less mass they have, the larger they are in diameter.
Andy Howell. Credit: UCSB
“This surviving star is a bit like Obi-Wan Kenobi returning as a ghost of force in Star Wars,” said co-author Andy Howell, an assistant professor at UC Santa Barbara and a senior scientist at Las Cumbres Observatory. “Nature tried to bring down this star, but it became more powerful than we could have imagined. It is still the same star, but it returns in a different way. Death transcended.”
For decades, scientists thought that type Ia supernovae explode when a white dwarf star reaches a certain size limit, called the Chandrasekhar limit, about 1.4 times the mass of the sun. This model has fallen a bit into disgrace in recent years, as many supernovae have been found to be less massive than this one, and new theoretical ideas have indicated that there are other things that make them explode. Astronomers were unsure whether the stars had approached the Chandrasekhar limit before exploding. The study authors now think that this growth to the final limit is exactly what happened with SN 2012Z.
“The implications for type Ia supernovae are profound,” McCully says. “We’ve found that supernovae can at least grow to the limit and explode. However, explosions are weak, at least sometimes. We now need to understand what makes a supernova fail and become an Iax type, and what it does. that a supernova is successful as type Ia “.
Reference: “Still Brighter than Pre-explosion, SN 2012Z Did Disappear: Comparing Hubble Space Telescope Observations a Decade Apart” by 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, February 1, 2022, The Astrophysical Journal.DOI: 10.3847 / 1538-4357 / ac3bbd