Hypernova Observations Reveal the Death of the Massive Stars

Hypernova Observations
Hypernova Observations
Credit: Anna Serena Esposito

The end of a star’s life can occur in a tranquil manner in the case of Low Mass stars, like the Sun. this is not the case, however, for very huge stars, that suffer such extreme explosive events that they can outshine the brightness of the entire galaxy that hosts them. an international group of astronomers has revealed a detailed study of the death of a high-mass star that produced a gamma-ray burst (GRB) and a hyper nova, in which they have detected a new component during this type of events. The study, revealed in Nature, provides a link that completes the scenario that relates hyper novae with GRBs.

“The 1st hypernova was detected in 1998 as a very energetic kind of supernova that followed a gamma-ray burst. This was the first proof of the connection between both phenomena” says Luca Izzo, researcher at the Institute of astrophysics of Andalusia (IAA-CSIC), and leader the study.

The scenario that has been proposed to explain the phenomena involves a star over twenty five times more huge than the Sun that, once it’s exhausted its fuel, suffers the collapse of its core. during this collapse, the nucleus of the star transforms either into a neutron star or a black hole, and at the same time, 2 polar jets of matter are ejected. These jets drill through the external layers of the star and, once out of the star, produce detectable gamma-rays (the so-called GRB). Finally, the external layers of the star area unit ejected, generating a hyper nova explosion, tens of times brighter than a typical supernova.

Although the connection between GRBs and hyper novae has been well established over the last twenty years, the opposite isn’t so clear, since there have been several hyper novae that don’t have associated GRBs. “This work has allowed us to seek out the missing link between these 2 types of hypernova through the detection of an additional component: a sort of hot cocoon generated around the jet, as it propagates through the outer layers of the progenitor star – indicates Dr. Izzo (IAA-CSIC) -. The jet transfers a significant part of its energy to the cocoon and, if it manages to reach the surface of the star, will produce the gamma-ray emission that we know as a GRB”.

Hypernova Observations Reveal
Gran Telescopio Canarias

On the other hand, the jet can fail to pierce the external layers of the star and ne’er emerge into the circumstellar medium if it lacks the necessary energy. during this case we’d observe a hypernova but not a GRB. The cocoon detected in this study is the link between the 2 subtypes of hypernovae that had been studied until now, and the chocked jets would naturally justify the observed differences.

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On December 5, 2017, GRB 171205A was detected in a galaxy located five hundred million light years from Earth. however far this may appear, this makes it the fourth nearest long GRB ever observed. “Such events occur on average every 10 years, so we now started an intense observing campaign to observe the emerging hyper nova from the very early phases on – says Christina Thöne, researcher at the Institute of astrophysics of Andalusia (IAA-CSIC) who participated in the discovery -. In fact, with our early observations we managed to get the earliest detection of a hyper nova to date, less than one day after the collapse of the star”.

And indeed, very early on the first features of a hyper nova were detected with the gran Telescopio Canarias, on the island of la Palma. “This was only possible because the luminosity of the jet was much weaker than usual, as typically the jets outshine the hyper nova during the first week – says Antonio de Ugarte Postigo, researcher at the Institute of astrophysics of Andalucia (IAA-CSIC) who participated within the paper -. What we saw, however, was a very peculiar component, which showed unprecedented expansion velocities and chemical abundances that were totally different to the ones seen in similar events”.

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This peculiar chemical composition and the high expansion velocities matched the expectations for the existence of a cocoon accompanying the jet at the surface of the star. This had been predicted but had been ne’er observed before. The cocoon observed during the first days dragged material out from the interior of the star, and its chemical composition was determined in this study. after some days, this component faded away, and the hyper nova evolved in a similar manner as the ones previously observed.

The total energy emitted by the cocoon during these 1st days was larger than that of the GRB, implying that the jet transferred a large part of its energy to the cocoon. However, it also indicates that the energy of the GRB depends to some degree on the interaction between the jet and the stellar material, and on this new component, the cocoon. This discovery also implies that the models must be revised: “While in the standard model of super nova the collapse of the nucleus ends up in quasi-spherical explosions, the proof of such an energetic emission produced by the cocoon suggests that the jet plays a vital role in core-collapse supernovae which means we’ll need to consider it in super nova explosion models”, concludes Izzo (IAA-CSIC).

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Publication: L. Izzo, et al., “Signatures of a jet cocoon in early spectra of a supernova associated with a γ-ray burst,” Nature volume 565, pages 324–327 (2019)

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