Astronomers Observe New Type of Supernova

The first case of a new kind of supernova was recently observed by an international team of astronomers.

Confirming a prediction made 40 years ago, this latest discovery may shed new light on the life and death of stars. The study was published in the Nature Astronomy journal on June 28^th, 2021.

One of the main questions in astronomy is to compare how stars evolve and how they die. There are many links still missing, so this is very exciting.

Stefano Valenti, Professor of Physics and Astronomy, University of California, Davis

Professor Valenti is also a member of the team that identified and explained the supernova 2018zd.

There are two familiar kinds of supernovae. A core-collapse supernova takes place when a giant star, measuring over 10 times the mass of the Sun, runs out of fuel, which causes its core to disintegrate into a neutron star or black hole. A thermonuclear supernova takes place when a white dwarf star — the remnants of a star measuring up to eight times the mass of the sun — blows up.

Back in 1980, Ken’ichi Nomoto from the University of Tokyo estimated a third type of supernova, known as an electron-capture supernova.

The energy created in the central core of a majority of the stars keeps them from disintegrating under their own gravity. In the case of an electron-capture supernova, as the central core runs out of fuel, electrons within the core are forced into their atomic nuclei by gravity, causing the star to disintegrate on itself.

Evidence from Late Spectrum

In March 2018, the supernova 2018zd was identified approximately three hours after the explosion. The Spitzer Space Telescope and the Hubble Space Telescope provided archival images that revealed an indistinct object that could be the star before the explosion. Comparatively close to Earth, the supernova is located at a distance of around 31 million light-years in the NGC2146 galaxy.

The research team, headed by Daichi Hiramatsu, a graduate student from UC Santa Barbara and Las Cumbres Observatory, obtained data on the supernova over the next couple of years.

Astronomers from the University of California, Davis (UC Davis), including Valenti and graduate students Azalee Bostroem and Yize Dong, provided a spectral analysis of the supernova about a couple of years after the explosion. This was one of the lines of proof demonstrating that 2018zd was an electron-capture supernova.

“We had a really exquisite, really complete dataset following its rise and fade,” stated Bostroem. That comprised very late data obtained with the 10-m telescope at the W.M. Keck Observatory based in Hawaii, added Dong.

Theoretically, electron-capture supernovae are expected to display a rare stellar chemical spectrum many years later.

The Keck spectra we observed clearly confirm that SN 2018zd is our best candidate to be an electron-capture supernova.

Stefano Valenti, Professor of Physics and Astronomy, University of California, Davis

The late spectral data was not the only piece of the mystery.

The researchers sifted through all the available information on supernovae and observed that while a few supernovae had some of the indicators estimated for electron-capture supernovae, only the SN 2018zd had all six — an evident progenitor star of the Super-Asymptotic Giant Branch (SAGB) type, a rare stellar chemical spectrum, powerful pre-supernova mass loss, little radioactivity, a weak explosion and a core rich with neutrons.

“We started by asking ‘what’s this weirdo?’ Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario,” stated Hiramatsu.

Explaining the Crab Nebula

These latest findings also shed light on certain mysteries of the most popular ancient supernova. In A.D. 1054, a supernova took place in the Milky Way galaxy. As per Chinese records, this supernova was so bright that it could be visualized at night for almost two years and in the daytime for 23 days.

The Crab Nebula — the resulting remains — has been investigated thoroughly. Previously, it was the best candidate for an electron-capture supernova, but this event was vague partly because the explosion occurred almost 1000 years ago. Now, the latest result boosts the confidence that the event that resulted in the formation of the Crab Nebula was an electron-capture supernova.

I am very pleased that the electron capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago. This is a wonderful case of the combination of observations and theory.

Ken’ichi Nomoto, Study Author, University of Tokyo

The study is part of the Global Supernova Project, headed by Professor Andrew Howell from UCSB and Las Cumbres Observatory.

Other co-authors of the study are Curtis McCully and Jamison Burke from the Las Cumbres Observatory and UCSB; Jared Goldberg and Chengyuan Xu from UCSB; Schuyler Van Dyk and Gagandeep Anand from the California Institute of Technology; Keiichi Maeda from Kyoto University; and Takashi Moriya from the National Astronomical Observatory of Japan.

Nozomu Tominaga from Konan University, Kobe, Japan; Griffin Hosseinzadeh from the Center for Astrophysics, Harvard & Smithsonian; Iair Arcavi from Tel Aviv University, Israel; Peter Brown from Texas A&M University; Jennifer Andrews, Christopher Bilinski, G. Grant Williams, Paul Smith, Nathan Smith, and David Sand from Steward Observatory, the University of Arizona were also involved in the study.

Other study co-authors are Alexei Filippenko from UC Berkeley; Melina Bersten and Gastón Folatelli from the Instituto de Astrofísica de La Plata and Universidad Nacional de La Plata, Argentina; Patrick Kelly from the University of Minnesota; Toshi- hide Noguchi from the Noguchi Astronomical Observatory; and Koichi Itagaki from the Itagaki Astronomical Observatory in Japan.

The study was partly funded by grants from NASA and the National Science Foundation

Journal Reference:

Hiramatsu, D., et al. (2021) The electron-capture origin of supernova 2018zd. Nature Astronomy. doi.org/10.1038/s41550-021-01384-2.