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Tuesday, 28 January 2020

A giant star devoured his companion and caused one of the brightest supernovas of all time



When a sufficiently massive star reaches the end of its life, it produces one of the most cataclysmic events in the Universe: a supernova. The light and energy released are such that the event often overshadows all the surrounding objects. However, there is an even more intense phenomenon: hypernovas (or superluminous supernovas). This is what astrophysicists observed in the constellation of Perseus 14 years ago, during the event called SN 2006gy. And recently, they proposed a hypothesis explaining the origin of the phenomenon.



In September 2006, “a” star 50 billion times brighter than the Sun, and in the process of exploding, located 240 million light years away in the constellation of Perseus, began to shine intensely. For 70 days, the explosion became more and more brilliant, eclipsing its original galaxy by being tens or even hundreds of times more powerful than a typical supernova. At the time, this super bright supernova (also known as a hypernova) was the brightest star explosion ever detected.

What was special about this record explosion (officially named SN 2006gy)? Nobody knew. But now, more than a decade later, astrophysicists may finally have an idea. In a new study published in the journal Science , the researchers re-analyzed the mysterious emission lines emanating from the explosion about a year after its peak.

The SN 2006gy event observed by the Hubble space telescope. Credits: Hubble

A hypernova involving a binary stellar system

The team discovered large amounts of iron in the emissions, which they said could only be the result of the interaction of the supernova with a preexisting layer of stellar material ejected hundreds of years earlier. Where does all this ejected stellar material come from? A likely scenario is that the SN 2006gy event did not start with one star, but with two.

“No one had tried to compare the spectra of neutral iron, that is to say the iron that all the electrons kept, with the unidentified emission lines of SN 2006gy, because iron is normally ionized. We tried it out and saw with enthusiasm how, line after line, the lines were aligned with the observed spectrum,” explains Anders Jerkstrand, of the University of Stockholm.

Many binary systems involve a giant star and a white dwarf. In the SN 2006gy scenario, the white dwarf accelerated matter from the giant companion star while spiraling towards it after being caught in the expansion of its gaseous envelope. Credit: Pearson Ed

"A candidate scenario to explain this is the evolution of a binary progenitor system, in which a white dwarf spirals towards a giant or supergiant companion star" explain the researchers.

Collisions between binary stars (two stars orbiting each other) are rare, occurring once every 10,000 years or so in the Milky Way. When the stars collide, they can eject into the surrounding space a gaseous envelope of stellar material when the two stellar nuclei slowly merge.

The progenitor of SN 2006gy was, according to the new model, a double star composed of a white dwarf the same size as Earth and a massive star rich in hydrogen as large as our Solar System, in close orbit. As the hydrogen-rich star has expanded its envelope, which occurs when nuclear reactions are triggered in the later stages of evolution, the white dwarf has been caught in the envelope and has spiraled towards the center of the companion.


A supernova amplified by the previous collision of the two stars

When it reached the center, the unstable white dwarf exploded and a so-called type Ia supernova was born. This supernova then collided with the ejected envelope, which is projected during the spiral descent of the white dwarf, and this gigantic collision gave birth to the light of SN 2006gy.

Artist's impression showing what the SN 2006gy hypernova could have looked like. Credits: NASA / CXC / M.Weiss

If such a collision had occurred between 10 and 200 years before the supernova was detected, the two stars could have released a gaseous envelope that lingered around the system when the stars merged over the next century. When the fusion finally ended with an explosion of type Ia supernova, the gaseous envelope could have amplified the brightness of the explosion to the staggering levels that astronomers observed, and also produced the appropriate iron emission lines. .

This explanation is, for the moment, purely mathematical, astrophysicists having never seen two binary stars merge. A new clue could come from a nearby star system called Eta Carinae.

Located about 7,500 light years from Earth, Eta Carinae is a pair of giant stars that have exploded slowly over the past few hundred years, gradually lighting up to become the brightest star system in the Milky Way. Researchers believe that stars could end up in hypernova in the next 1000 years.




Bibliography:

A type Ia supernova at the heart of superluminous transient SN 2006gy

Anders Jerkstrand, Keiichi Maeda, Koji S. Kawabata

Science  24 Jan 2020

Vol. 367, Issue 6476, pp. 415-418

DOI: 10.1126/science.aaw1469

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