The missing piece

Debarati Chatterjee
Jan 27, 2019
2 min read

The recent observation of a hypernova solved the age old mystery involving the death of massive stars.

Stars more massive than the sun often end their lives in a cataclysmic event, called a supernova, in which the inner core collapses to form a compact object, while the outer layers of the star bounce and explode into the interstellar medium. In certain cases, these explosions are ten times more energetic, resulting in a hypernova. In a few events, associated high energy jets ejected along their poles were observed in high energy gamma-ray frequencies, also known as Gamma Ray Bursts (GRBs). However, the connection between the two events could not definitively be established as several hypernovae were observed without associated GRBs.

The breakthrough came recently (Dec 5, 2017) with the observation of

GRB 171205A by the research group led by Dr. Luca Izzo at the Institute of Astrophysics of Andalusia (IAA-CSIC). Immediately after the collapse of the star, intense observations were carried out with the Gran Telescopio Canarias, to follow the hypernova event right from the very early phases. During this phase, the jets are not strong enough to outshine the hypernova, and the observed peculiar chemical abundances and high expansion velocities established the existence of a cocoon due to the interaction of the GRB jet with the outer layer of the stellar surface, that expelled matter from the interior. In the following days, this component became less dominant and the evolution of the hypernova was similar to previously observed ones.

In order to understand supernova events, scientists construct theoretical models and then compare them with astrophysical observations. In existing models, the stellar collapse during the explosion is assumed to be spherical. However in the recent hypernova observation, the energy of the GRB was found to depend on the interaction between the jet, the stellar material as well as the cocoon. This interaction of the jets with the outer stellar layers should then be incorporated into the current supernova models.