Observations of a rare hypernova: completing the picture of the death of the most massive stars

  • A work led by the Institute of Astrophysics of Andalusia (IAA-CSIC) studies in detail the death of a massive star
  • The work, published in Nature, reports on a gamma-ray burst (GRB) and a hypernova arisen from this cosmic event
  • The discovery implies that the previously existing models about hypernovae will need to be revised

The deaths of stars

The end of the life cycle of a star can happen quietly in the case of low mass stars, such as the Sun. This is not the case, however, for very massive stars. These stars suffer extreme explosive events that can even outshine the brightness of the galaxy hosting them.

An international group of astronomers have done a detailed study of the death of a high-mass star that produced a gamma-ray burst (GRB) and a hypernova. Their research found a new component in this type of events. The study, published in Nature, provides a link completing the scenario that relates hypernovae with GRBs.

The study was coordinated by researchers of the High-Energy Transients and their Hosts (HETH) research group at IAA-CSIC. This group, led by Dr. Christina Thöne, studies the physics of transient astronomical phenomena, the environment in which they are produced and the galaxies that host them.

“The first hypernova was detected back in 1998 as a type of very energetic supernova that followed after a gamma-ray burst. This was the first evidence of the connection between supernovae and gamma ray bursts” says Luca Izzo, researcher of the HETH group.

Representation of hypernova showing the jets (diagonal axis) forming a cocoon surrounding the jet heads at the star surface
Representation of hypernova showing the jets (diagonal axis) forming a cocoon surrounding the jet heads at the star surface

The scenario proposed to explain the phenomenon involves a star over 25 times more massive than the Sun. Once that star has exhausted its fuel, it suffers the collapse of its core. During the collapse, the nucleus of the star transforms either into a neutron star or a black hole. At the same time, two 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 mentioned GRB). The external layers of the star are finally ejected due to the jets, generating a hypernova explosion, tens of times brighter than a typical supernova.

The direct connection between GRBs and hypernovae has been well established over the last 20 years. The opposite, however, is not so clear, as there have been several hypernovae that do not have an associated GRB. We can hence make a distinction between two types of hypernovae, depending on whether they display or not a detectable gamma ray burst.

“This work has allowed us to find the missing link between these two 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, produces the gamma-ray emission that we know as GRB.”

In some cases, the jet can fail to pierce the external layers of the star and never emerge into the circumstellar medium if it lacks the necessary energy. It is in such circumstance that we would observe a hypernova but not a GRB. So, the cocoon detected in this study is the link between the two subtypes of hypernovae that had been studied. The “choked” (or “limited”) jets would explain the observed differences.

The event (or the story of a quick reaction)

On December 5, 2017, GRB 171205A was detected in a galaxy located 500 million light years from Earth. However far this may seem, this makes it the 4th closest long GRB ever observed. “Such events occur on average every ten years, so we immediately started an intense observing campaign to observe the emerging hypernova 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 obtain the earliest detection of a hypernova to date: less than one day after the collapse of the star”.

Very early on, the first features of a hypernova 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 hypernova during the first week – says Antonio de Ugarte Postigo, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participated in the paper –. What we saw, however, was a very peculiar component, which showed unprecedented expansion velocities and chemical abundances that were different to the ones seen in similar events.”

Image of the explosion obtained by the Gran Telescopio Canarias during the period of maximum brightness of the event.
Image of the explosion obtained by the Gran Telescopio Canarias during the period of maximum brightness of the event.

The peculiar chemical composition and high expansion speeds matched the expectations for the existence of a cocoon accompanying the jet on the surface of the star. This had been predicted but had been never 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 a few days, this component faded away, and the hypernova evolved in a way similar to those observed in the past.

The total energy emitted by the cocoon during the first days was larger than that of the GRB. This implies 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 the new component: the cocoon.

This discovery implies that the existing models must be revised: “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”, concludes Izzo (IAA-CSIC).

Image credits:

All images kindly provided by the Instituto de Astrofísica de Andalucía. Artistic image of hypernova explosion displaying the jets, by Anna Serena Esposito.