The Hubble Space Telescope captured this dramatic spiral galaxy, cataloged as NGC 1309. Credit: NASA, ESA, Hubble Heritage Team, (STScI / AURA) and A. Rhys (STScI)
A stubborn star confirms a reworked model of supernovae
The supernova is the cataclysmic explosion of a star. In particular, thermonuclear supernovae signal the complete destruction of a white dwarf star, leaving nothing behind. At least that’s what astrophysical models and observations suggested.
So when a team of astronomers went to study the location of the special thermonuclear supernova SN 2012Z with the Hubble Space Telescope, they were shocked to find that the star survived the explosion. Not only had she survived, but the star was actually even brighter after the supernova than before. First author Curtis McCully, a postdoctoral fellow at the University of California, Santa Barbara and the Las Cambres Observatory, presented the 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 one of the most important instruments in astronomers’ tools for measuring cosmic distances. Beginning in 1998, observations of these explosions revealed that the universe was expanding at an ever-accelerating rate. This is believed to be due to the dark energy, the discovery of which won the Nobel Prize in Physics in 2011.
Left: Color image of the galaxy NGC 1309 before Supernova 2012Z. Right: Clockwise from top right: the position of the supernova before the explosion; SN ~ 2012Z during the visit in 2013; the difference between the images before the explosion and the observations from 2016; the location of SN ~ 2012Z in the last observations in 2016. Credit: McCully et al.
Although they are vital to astronomy, the origins of thermonuclear supernovae are poorly understood. Astronomers agree that they are destroying white dwarf stars – stars with approximately the mass of the sun collected in the size of the Earth. What makes the stars explode is unknown. One theory is that the white dwarf steals matter from another star. When the white dwarf becomes too heavy, the thermonuclear reactions ignite in the nucleus and lead to a rapid explosion that destroys the star.
Curtis McCully. Credit: UCSB
SN 2012Z was a strange type of thermonuclear explosion, sometimes called a Iax supernova. They are the weaker and weaker cousins of the more traditional Type Ia. Because they are less powerful and slower explosions, some scientists theorize that they are unsuccessful type Ia supernovae. New observations confirm this hypothesis.
In 2012, the supernova 2012Z was discovered in the nearby spiral galaxy NGC 1309, which was studied in depth and captured in many images of Hubble in the years leading up to 2012Z. Hubble’s photos were taken in 2013 in a concerted effort to identify which star in the older images matched the star that exploded. The analysis of these data in 2014 was successful – scientists were able to identify the star in the exact position of the supernova 2012Z. This was the first time a supernova white dwarf ancestor had been identified.
“We were expecting to see one of two things when we got the latest Hubble data,” McCully said. “Either the star would disappear completely, or maybe it would still be there, which means that the star we saw in the images before the explosion was not the one that exploded. No one expected to see a surviving star brighter. It was a real puzzle. “
McCully and the team believe that the half-exploded star has become brighter because it has swelled to a much larger state. The supernova was not strong enough to blow all the material, so part of it fell back into what is called a bound residue. Over time, they expect the star to slowly return to its original state, only less massive and larger. Paradoxically, for white dwarf stars, the smaller their mass, the larger their diameter.
Andy Howell. Credit: UCSB
“This surviving star is a bit like Obi-Wan Kenobi, who is returning as a ghost of power in Star Wars,” said co-author Andy Howell, an associate professor at the University of California, Santa Barbara and a senior scientist at the Las Cambres Observatory. “Nature tried to bring down this star, but it came back more powerful than we could have imagined. It is still the same star, but again in a different shape. It transcends death. “
For decades, scientists have believed 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 somewhat out of favor in the last few years, as many supernovae have been found to be less massive than this one, and new theoretical ideas show that there are other things that make them explode. Astronomers were unsure if the stars had ever approached the Chandrasekhar boundary before exploding. Now the authors of the study believe that this growth to the limit is exactly what happened with SN 2012Z.
“The consequences for type Ia supernovae are profound,” McCully said. “We’ve found that supernovae can at least grow to the limit and explode. Still, the explosions are weak, at least in some cases. Now we need to understand what makes a supernova fail and become type Iax and what makes a successful type Iax.
Reference: “Still brighter than before the explosion, SN 2012Z has not disappeared: Comparing the observations of the Hubble Space Telescope a decade away” by Curtis McCully, Surab W. Ja, Richard A. Scalzo, D. Andrew Howell, Ryan J. Foley, Yaotiang 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 / ac
Add Comment