A star survived its own supernova, and no one knows why

This “zombie star” seems to have miraculously survived the gigantic thermonuclear explosion that usually marks the death knell for white dwarfs. But why ?

Astronomers recently made an extremely surprising discovery: they spotted the trace of a white dwarf which seems to have survived its own supernova, which could call into question a good part of the knowledge of specialists on this phenomenon.

During its life, a star goes through different stages. The so-called main sequence stars, like the sun, are the site of thermonuclear reactions of terrifying power. Once the fuel essential to these reactions has been exhausted, they can take two distinct paths depending on their mass.

White dwarfs, stars at the end of their life

The heavier ones become so-called supergiant stars. The majority of them, on the other hand, end up as red giants. The latter continue to swell and gradually cool, until the entire peripheral part of the star is ejected; we then obtain nebulae which leave behind them white dwarfs. In essence, these are the hearts of these stars at the end of their course.

But if they are chemically more or less dead, these white dwarfs are not not inert however. They are extremely massive objects compared to their relatively modest size; and this particularity is at the center of a fundamental phenomenon of cosmology.

An artist’s impression of a white dwarf pulling surrounding material towards it. © University of Warwick – Mark Garlick

Like all such heavy objects, white dwarfs exert a monstrous gravitational force. They tend to pull all neighboring celestial bodies out of their respective orbits. This results in a gargantuan feast; for a very short period on the scale of the life of the star, the latter gorges on all this material.

It is thus moving towards a threshold called “critical mass”; once exceeded, the structure of this now obese star is no longer stable. It ends up disappearing in a thermonuclear explosion of Dantesque proportions. We then speak of a supernova (in this case, a type Ia supernova)… or at least, this is the scenario that was more or less unanimous until astronomers get to know 2012Z.

A zombie star that survived its own supernova

This number designates a supernova spotted in 2012 in a galaxy called NGC 1309, 120 million light-years from Earth. And luckily, it turns out that the venerable Hubble had already taken the portrait to this corner of the sky in the past. The researchers therefore reopened the telescope archives to try to find the trace of this event.

This allowed them to identify the precise system where it took place; they spotted a white dwarf especially voracious cannibalizing a nearby star, presumably a red giant. An object that looked very much like the perfect candidate

The NGC 1309 galaxy, the scene of this spectacular discovery. © NASA / ESA / Hubble Heritage Team (STSCI/AURA) / A. Riess (JHU/STSCI)

This observation was already very impressive in many ways. It was in particular the very first time that scientists have unearthed an image of a white dwarf before the fateful explosion. But the more the researchers progressed in interpreting the data, the more a new mystery seemed to emerge.

Indeed, against all expectations, the most recent images all pointed in the same direction: not only would the famous white dwarf be still there after this apocalyptic eventbut it would even be brighter than before ! What raise the eyebrows of all the researchers involved. ” No one expected to see an even brighter surviving star “, concedes the American astronomer Curtis McCully in a press release.

A unique example in the scientific literature

Is Hubble starting to squint? Could a big error have been hidden in the specialists’ interpretation? After numerous checks, these two outcomes now seem very improbable and the researchers favor another track; according to them, this white dwarf would in fact be a “ zombie star from a supernova gone wrong.

Other astronomers have already mentioned supernovae of this type (we speak of Iax supernovae) in the specialized literature. But so far, no one had managed to confirm the survival of a star with images before and after the cataclysm. From now on, the challenge will therefore be to determine how did she get away with it so cheaplybut also de determines the origin of its unusual brilliance.

The brilliance of this white dwarf leaves astronomers perplexed. © McCully et al.

The researchers also have their own idea about the sequence of events. According to them, the supernova would have been much less powerful than one would expect in this case. This means that the cloud of material from the supernova (we speak of remnant) was ejected with less force than usual.

Usually these slash can travel a considerable distance. But here he wouldn’t even have had enough traction to escape the grip. In all likelihood, it would therefore have quickly retraced the opposite path towards the white dwarf. He would then have remained a prisoner under the effect of gravitation. This superheated remnant taken hostage by the star would be the main source of this surprising luminosity.

The researchers suggest that the signal picked up by Hubble could also be amplified by the brightness of the neighboring star, which was also overheated by the supernova. The rest of the luminosity would come from radioactive material ejected into interstellar space.

On the other hand, these tracks do not make it possible to shed light on all the gray areas. There are still inconsistencies at the chronological level, since this luminosity also seems to decrease much more slowly than expected.

Deep implications in fundamental cosmology

But above all, the phenomenon responsible for this aborted supernova remains as mysterious as ever. And the researchers intend to reach the end of the story. ” Now we need to understand what causes a Type Iax supernova to fail, and what defines a normal Type Ia supernova. says McCully.

And it’s not just for the sake of racking your brains. “ Implications for Type Ia supernovae are profound “, hammers the astronomer. Indeed, the variations in luminosity of supernovae are fundamental data in the study of the cosmos. They serve as landmarks that allow astronomers tostudy some of the most obscure and interesting phenomena of our worldsuch as the expansion of the universe or the famous dark energy which would represent almost 70% of the universe.

For researchers, it is therefore very important to control all the factors that can vary this luminosity, like these aborted Iax-type supernovae. They will therefore continue to work on this unique cosmological curiosity.

The text of the study is available here.

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