An article published in the journal “Nature Astronomy” reports a study on the supernova 2018zd. A team of researchers studied observations conducted with various telescopes and concluded that this supernova is of the electron-capture type, which was theorized 40 years ago but never identified with certainty. According to the researchers, 2018zd matches the theoretical predictions and offers new confirmations to the hypothesis that the supernova observed in A.D. 1054 was of the electron-capture type.
There are various types of supernovae, cosmic explosions caused by various mechanisms in stars of different types. About 40 years ago, Professor Ken’ichi Nomoto of the University of Tokyo proposed with some collaborators a mechanism in which particular stars, in the super-asymptotic giant branch (SAGB), when certain conditions are met, can explode into supernovae different from the ones already known. A certain density is needed for the protons of the magnesium atoms that form the degenerate nucleus of that type of star to start capturing part of the electrons that counteract the pressure, preventing the nucleus from collapsing. The consequence is that the pressure of the star’s mass is no longer compensated and the processes that cause the supernova are triggered.
Even on a theoretical level, there were uncertainties in the predictions of the characteristics of a supernova of the type that was named electron-capture, which made it even more difficult to find one. Now researchers including Professor Nomoto himself believe they have found one.
Over the years, simulations have been conducted to try to identify the characteristics of an electron-capture supernova and thus be able to recognize one when it is observed. The results indicate that the star loses a lot of mass, which remains nearby and has a certain chemical composition. The loss of mass makes the supernova weaker than those of other types, emits a limited amount of radiation, and has neutron-rich elements in its nucleus.
The supernova 2018zd was identified thanks to the telescopes of the Las Cumbres Observatory and immediately several unusual features were noticed, some never observed in known supernovae. Its origin is close to the galaxy NGC 2146, a location that allowed researchers to examine pre-explosion archival images captured by the Hubble Space Telescope to find the progenitor star that likely generated that supernova.
The image (NASA/STScI/Joseph Depasquale; Las Cumbres Observatory) shows a composition of observations conducted by the Las Cumbres Observatory and the Hubble Space Telescope that includes the supernova 2018zd, in the white circle on the right, near the galaxy NGC 2146.
The likely progenitor star of the supernova 2018zd has similar characteristics to a recently identified super-asymptotic giant branch star. The supernova has the characteristics theorized for an electron capture type. This identification offers information on the processes that occur in the final phase of life of a certain type of star with a mass between 8 and 10 times the Sun’s.
The discovery of an electron-capture supernova confirms the hypothesis that even the one sighted in A.D. 1054 was of that type. It’s not easy to draw conclusions based on historical descriptions dating back almost a millennium but it was an exceptional supernova, visible even during the day for 23 consecutive days and at night for almost two years. Its remnants, which formed the Crab Nebula, help this “space archeology” task. The set of data collected helps to understand how some neutron stars are formed and the formation of chemical elements that are projected into interstellar space and can help form new planets.
