It appears that some of the extremely massive “Wolf-Rayet stars” could end their lives by exploding in supernovae. Until then, they were thought to have a “silent” end, collapsing into black holes after expelling their outer layers. The discovery was made by a team of researchers, who analyzed the spectrum of the supernova SN 2019hgp, detected by the Zwicky Transient Facility (ZTF) survey.
During the analysis, the team found that the supernova’s spectrum had some bright emissions, which indicated the presence of elements such as carbon and oxygen, but not hydrogen or helium. By poring over the data, the authors found that the emission lines did not come from the supernova elements, but rather from a nebula expanding away from the star at a speed of 1,500 km/s.
This means that, before the supernova happened, the star was surrounded by a nebula rich in carbon and nitrogen, without the presence of lighter elements. Thus, the nebula’s expansion must have been caused by strong stellar winds, which matches the already known structure of Wolf-Rayet stars. Therefore, it is possible that SN 2019hgp is the first example of a “Wolf-Rayet supernova”. Since then, similar supernovae have been detected.
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A new type of supernova?
Wolf-Rayet stars are considered to be some of the most massive we know of. They are short-lived compared to other stars, and when they reach the end of their lives, they push their outer layers through a strong stellar wind that produces a nebula rich in ionized helium, carbon, nitrogen, but very little hydrogen.
Even after expelling the outer layers, the “leftover” central star is still much more massive than the Sun. Therefore, one would expect it to consume all the fuel of nuclear fusion until it explodes in a supernova. However, in the case of Wolf-Rayet stars, there is a difference: the spectrum of elements from supernovae can be observed, but it has not yet been possible to observe one that corresponds to that of the Wolf-Rayet star. So astronomers began to question whether it was possible for them to die a silent death.
In that case, after expelling the layers, the remaining core would collapse directly into a black hole. As SN 2019hgp was identified through the spectrum of a nearby nebula, it is still unclear whether the explosion was, in fact, an ordinary supernova or if there was something else, perhaps happening in the upper layer of the star that exploded, while the core collapsed into a black hole.
The article with the results of the study was published in the arXiv online repository, without peer review.
