A white dwarf survived its supernova explosion by becoming a “zombie” star

In about two weeks, the Nasa will make public the first spectroscopic images and data from James Webb Space Telescope allowing to begin the collection of scientific information with this new eye on orbit of the noosphere. But the Hubble telescope, its predecessor, has yet to say its last word, either with new observations or by analyzing in a new way those it has made in the past and which are archived for future generations. ofastrophysicists.

On this last point, a new proof has just been given by Curtis McCullya postdoctoral researcher at UC Santa Barbara and the Las Cumbres Observatory, who published with his colleagues an article in the renowned The Astrophysical Journal. Available on arXivit contains results from observations carried out with Hubble during the years 2000-2010 and which were presented at a press conference on the occasion of the 240^e meeting of the American Astronomical Society.

The researchers announced there that they had confirmed the existence of a new type of thermonuclear supernovae, a variant of type supernovae NS Ia produced with white dwarfs and which are called SN Iax. Less powerful than their cousins, but more than simple novae, they do not lead to the destruction of their star parent and it is a surprise that blurs our understanding of supernovae with white dwarfs.

The SN Iax occurred in January 2012, in the form of the supernova SN 2012Z having been detected within the framework of the research program Lick Observatory Supernova Searchin spiral galaxy neighbor of the Milky Way, NGC 1309.

A supernova zoo

But before examining this discovery more closely, let’s take a look at some explanations already given by Futura in a previous article. It concerns what has been called for several centuries since Tycho Brahe and Johannes Keplerfamous builders of Heaven, “new stars”, abbreviated to novae, from latin stella-nova which means “new star”. But it was not until the 1930s and the work of Walter Baade and Fritz Zwicky that we realized the differences between novae and supernovae. Quickly, were also discovered the first subclasses of supernovae today famous, the SN I and the SN II of the famous classification designed by theastronomer German-American Rudolph Minkowski and Swiss astronomer Fritz Zwicky.

SN Ia are thermonuclear explosions of white dwarfs in binary systems while the much more powerful SN IIs are explosions produced by stars much heavier than the Sun and which collapse gravitationally giving neutron starsor some black holes if they are big enough. In any case, the differences between supernovae (others were going to be highlighted until our days still), are at the level of the spectrum reflecting the presence of certain elements in the light explosions and in the variations and durations luminous intensities (curves of light) of these stellar cataclysms.

Classic SN Ia

Thus, type I supernovae have a spectrum that does not contain hydrogen while type II supernovae have a spectrum that does. Among type I supernovae, three subclasses are distinguished, so that if the spectrum shows the presence of silicon, we speak of type Ia but if the spectrum does not show any, we look at the abundance of helium. In the presence of a significant quantity of He, one speaks of type Ib and conversely, in the presence of a small quantity of helium, one speaks of type Ic.

The SN Iax type has therefore just been introduced for a few years now.

The classic model for an SN Ia was the following. It all starts in a binary system where a star a little more massive than the other, but not exceeding 8 to 10 masses solar cells, evolves more rapidly by first becoming a red dwarf. This leads her to lose mass with winds violent, to end up leaving a stellar corpse in the form of a white dwarf containing less than 1.44 times the mass of the Sun.

If the two stars are close enough to each other when the second one in turn becomes a red giant, the tidal forces gravity of the first snatch mass from it. It forms a accretion disk around the white dwarf which sees its mass increase by swallowing, to form its outer layers, hydrogen and helium while its core contains a lot of carbon and oxygen. A series of thermonuclear reactions start racing when the mass of the white dwarf reaches 1.44 times that of the Sun and a thermonuclear explosion then occurs, completely destroying the white dwarf and leaving a supernova remnantas can be seen in the animation in the video above.

A zombie star produced by an SN Iax

This picture began to blur about a decade ago. It was first suspected that some SN Ia are in fact sometimes collisions of white dwarfs and, in fact, observations support this scenario.

Finally, for some years, after being able to evaluate the mass of white dwarfs spawning supernovae SN Ia, it was discovered that it was lower than the mace of chandrasekharso that it has become necessary to review the mechanisms behind the thermonuclear explosion.

Surprisingly in the case of SN 2012Z, not only did the white dwarf survive an explosion in supernova mode but it became brighter as seen in the inset to the right of the photo above, suggesting an increase in size. This phenomenon has been described as a “zombie” star since it has somehow come back to life, while basically remaining the corpse of a star once on the main sequence.

A type Iax supernova is less bright and its light curve evolves more slowly, leading nuclear astrophysicists to believe that SN 2012Z was somehow a failed supernova, whose details of thermonuclear reactions behind the explosion are still poorly understood. . It’s really disturbing and, for the moment, it defies previous models of SN Ia because, in the case of SN 2012Z, the explosion did occur with a progenitor star that we could identify on images from the Hubble archives, and whose mass was close to that of Chandrasekhar. Better, it is the first time that one identifies the white dwarf progenitor of a supernova.

In the UC Santa Barbara statement accompanying the discovery, McCully concludes that “ the implications for Type Ia supernovae are profound. We found 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 “.