First supernova explosion of a Wolf-Rayet star discovered

IAC/DICYT An international study, involving researchers from the Gran Telescopio Canarias (GTC or Grantecan) affiliated with the Instituto de Astrofísica de Canarias (IAC), has discovered the first explosive star of its kind, whose existence, until now, was only theoretical. The finding is published in the journal Nature.

Until a few years ago, the discovery of a supernova – the explosion of a very massive star, usually at the end of its life – was considered a rare event. Today, advanced measurement instruments and analysis methods allow fifty such explosions to be detected every day, which has made it easier for researchers to detect anomalous types of explosions that until now only existed as theoretical constructs.

Recently, an international team of scientists, led by researcher Avishay Gal-Yam, from the Department of Particle Physics and Astrophysics at the Weizmann Institute, has discovered a supernova that has never been observed before. It is an explosion originating from a Wolf-Rayet star, a highly evolved type of massive star that suffers large mass losses due to intense stellar winds.

Evolution of the Wolf-Rayet stars

The core of every star is fueled by nuclear fusion, in which lighter elements fuse to form heavier elements. The energy produced inside the star maintains extremely high temperatures that cause its gaseous matter to expand, thus keeping a fine balance with the force of gravity that pulls the star’s mass toward its center. When the star runs out of elements to fuse and stops producing energy, this balance is broken, giving rise to a huge black hole, which makes it collapse on itself, or to the explosion of the star, which releases the elements into the Universe. fused during their evolution.

The life of massive stars is considered to be relatively short, a few million years at most. The Sun, by comparison, has a life expectancy of about 10 billion years. Subsequent nuclear fusion processes in the core of massive stars lead to their stratification, in which heavy elements concentrate in the core, and gradually lighter elements make up the outer layers.

Wolf-Rayet stars are especially massive stars that are missing one or more of the outer layers made up of lighter elements. In this way, instead of hydrogen – the lightest element – the surface of the star is characterized by the presence of helium or even carbon and heavier elements. A possible explanation for this phenomenon is that the strong winds that blow due to the high pressure in the envelope of the star, scatter its outermost layer, causing it to lose layer after layer for several hundred thousand years.

The first exploding star of its kind

Despite their relatively short lifespan and state of progressive disintegration, analysis of the growing number of supernova discoveries led researchers to suspect that Wolf-Rayet stars do not simply explode, but quietly collapse into black holes; otherwise, one could have been observed. This hypothesis, however, has just been dismantled thanks to the recent discovery.

Spectroscopic analysis of the light emitted by the explosion allowed the discovery of spectral signatures that are associated with specific elements. In this way, the researchers were able to show that the explosion contained carbon, oxygen and neon atoms. This last element had not been observed in any supernova to date. In addition, the researchers identified that the matter that emits the cosmic radiation did not itself participate in the explosion, but originated in the space surrounding the star that exploded. This, in turn, strengthened the hypothesis in favor of strong winds stripping the star of its outer envelope.

Since this observation is the first of its kind, Gal-Yam states that it may be too early to unequivocally determine the fate of all these stars: “We cannot say at this time whether all Wolf-Rayet stars end their lives with An explosion or not. It could be that some of them collapse silently into a black hole. “

The researchers estimate that the mass that was scattered during the explosion is probably equal to that of the Sun or a slightly smaller star. However, the star that exploded was significantly heavier, with a mass at least ten times that of the Sun, making scientists wonder where most of the mass went.

Gal-Yam suggests an intermediate scenario, in which both possible fates are fulfilled at the same time: once the nuclear fusion in the core of the star is exhausted, an explosion takes place that throws part of the mass into space, while the mass remaining collapses in on itself, forming a black hole. “One thing is certain, this is not the ‘silent collapse’ that was spoken of in the past”, highlights the researcher.

The study used observations made with different telescopes, including the Gran Telescopio Canarias (GTC or Grantecan) located at the Roque de los Muchachos Observatory (Garafía, La Palma). For Antonio Cabrera Lavers, head of scientific operations at Grantecan and an affiliate researcher of the IAC who participated in the study, “it is worth mentioning that, since this discovery was made, another similar explosion of a Wolf-Rayet star has been observed, which it implies that this phenomenon is not, in fact, a unique event ”.

For his part, David García Álvarez, co-author of the article and an astronomer from Grantecan affiliated with the IAC, considers that “it is possible that the better our detection and measurement instruments are, this type of explosions, today considered rare and exotic, will become something common”.

Antonio Marante Barreto, a telescope operator at Grantecan who has also participated in the observations, stresses: “Supernovae can seem like colossal events that occur very, very far away and do not have a direct impact on our lives, but, to tell the truth, they are at the heart of life itself; planet Earth and all its forms of life, including us, are the result of such an event ”.

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