A rare hypernova shows the possible death of a very massive star

An article published in the journal “Nature” reports the observation of a gamma-ray burst (GRB) cataloged as GRB 171205A associated with a supernova cataloged as SN 2017iuk that was tens of times brighter than that type of event generally is, so as to fall into the category of hypernovae. A team of astronomers led by Luca Izzo of the Instituto de Astrofísica de Andalucía (IAA-CSIC) detected for the first time interaction between the jet that caused the GRB and the outer layers of the exploded star. This allowed to better understand the mechanisms that combine hypernovae and gamma-ray bursts, connected to a “hot cocoon”.

When stars are about to explode, it’s normal for them to eject their outer layers, which are normally composed of hydrogen. In type Ic hypernovae, those layers contain many heavier chemical elements, even iron, which are ejected at speeds that can reach 30,000 km/s, one tenth of the speed of light. This happens in the agony of very massive stars, at least 30 times more massive than the Sun. When they reach the final collapse, two jets of materials are ejected from their poles with an energy that reaches the outside generating extremely strong electromagnetic emissions, in particular the gamma-ray burst. At that point the outer layers’ ejection takes place, followed by the hypernova.

Not all was clear about the mechanisms of the hypernova and the gamma-ray burst associated with it but new information were collected after GRB 171205A was detected by the Swift space telescope in a galaxy about 500 million light years from Earth on December 5, 2017. It’s one of the closest long gamma-ray bursts ever detected and is an event that takes place on average once every ten years. The consequence was that an intense observation campaign began immediately to study the hypernova from its very early stages, succeeding in obtaining the quickest detection of that type of phenomenon, less than a day after the progenitor star’s collapse.

The 10.4 meter Gran Telescopio Canarias, on the island of La Palma, and ESO’s VLT in Chile allowed to carry out in-depth observations that indicated that it was an event different from a common supernova. The detections of the chemical signatures of the ejected elements and their velocity matched the theoretical estimates of the existence of a cocoon that follows the jet emerging from the progenitor star’s photosphere. The cocoon brought materials out of the star’s interior but that component lasted only a couple of days and then the hypernova evolved in a way similar to the ones observed previously.

The top image (courtesy Antonio de Ugarte Postigo (IAA-CSIC). All rights reserved) shows the image taken by the Gran Telescopio Canarias of the hypernova SN 2017iuk. The bottom image (courtesy Anna Serena Esposito. All rights reserved) shows an artistic representation of the first phases of the type of explosion occurred in this hypernova.

A surprise came from the estimate of the energy emitted by the cocoon, which was higher than that emitted in the gamma-ray burst. This indicates that the jet transferred a considerable part of its internal energy to the cocoon, showing that in those gamma-ray bursts the energy depends mainly on the interaction between the jet and the progenitor star’s materials. In essence, the researchers identified an important role of the cocoon and in its interactions present in various type Ic and Ib supernovae and in hypernovae not associated with gamma-ray bursts. These events release enormous amounts of energy in both the gamma-ray burst and in the explosion so astronomers are very interested in their mechanisms.

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