A clearer picture of how massive stars die


An international study has revealed why some hypernovae do not emit GRBs. Nick Carne reports.


An artist's representation of the hypernova.

An artist's representation of the hypernova. The interaction of the jet with the outer layers of the star forms a cocoon that surrounds the head of the jet and begins to propagate laterally with respect to the direction of the jet.

Anna Serena Esposito

Astronomers have been given a glimpse of a massive star dying, providing a new piece to the puzzle of how it all happens.

In a paper published in the journal Nature, an international team reports on its detailed study of the death of a high-mass star that produced a gamma-ray burst (GRB) and a hypernova.

GRBs are the most powerful explosions in the cosmos, lasting several seconds and emitting as much light as nearly all the stars in the universe. Such extreme amounts of energy can only be released during catastrophic events such as the death of a very massive star, and also produce visible supernovae or hypernovae, which are five to 50 times more energetic than supernovae.

While the connection between GRBs and hypernovae has been well established, the researchers say, it has not been clear why some hypernovae do not have associated GRBs.

Through the detailed observation of a long-duration GRB and a rare nearby hypernova, the team led by Luca Izzo, from the Institute of Astrophysics of Andalusia in Spain, discovered the missing link in the form of a hot cocoon.

"For a handful of hypernovae not accompanied by GRBs, an excess of high velocity material had been observed in their optical spectra, which had been attributed to a thermal cocoon, namely a jet which never managed to break out of the interior to the surface of its progenitor star," says physicist Chryssa Kouveliotou from the University of Washington in the US.

"With this recent event, we showed the jet provided a significant part of its energy to the cocoon, which allowed both the jet and the gamma rays to break out of the surface of the star. This was the first time we actually got to peek directly at the core of a collapsing massive star."

The researchers discovered their GRB, now known as 171205A, in December 2017 in a spiral galaxy about 500 million light-years from Earth, making it the fourth closest long-duration GRB ever observed.

As it was so close and was detected early, they were able to monitor the evolution of the source daily, capturing an unprecedented level of information about the event in many different wavelengths over time.

Within days, evidence of the presence of a hypernova was reported, but it was peculiar. Hypernovae are characterised by high expansion velocities of around 30,000 kilometres per second, but this one (designated SN 2017iuk) exceeded 100,000 kilometres per second in the first few hours after the explosion.

"We first noticed a peculiar component, which showed very high velocity and unusual chemical composition not seen previously in similar events," Kouveliotou says. "These features fit perfectly with the assumption that we observed material from the central engine escaping from the GRB progenitor star.”

The team did in fact observe a hot cocoon taking material from the interior of the star to the outer layers. After about three days, the hot cocoon faded away and the hypernova began behaving similarly to those observed previously.

The researchers indicated the energy emitted by the cocoon in the earliest days of the hypernova was larger than that of the GRB, implying the jet deposited the bulk of its energy into the cocoon. These results demonstrate choked jets are indeed the reason some hypernovae do not seem to be associated with GRBs.

The researchers noted the findings have interesting consequences for how supernova and hypernova explosion models are constructed in the future.

"While in the standard model of supernovae the collapse of the nucleus leads to quasi-spherical explosions, the evidence of such an energetic emission produced by the cocoon suggests that the jet plays an important role in core-collapse supernovae which means we will need to consider it in supernova explosion models", Izzo says.

  1. https://www.nature.com/articles/s41586-018-0826-3
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