Scientists discovered a new type of supernova: an electron-capture supernova

The Goldilocks Supernova.

Supernovae are usually categorized into two main types: thermonuclear and iron-core collapse. A thermonuclear supernova is the explosion of a white dwarf star after it gains matter in a binary star system. An iron core-collapse supernova occurs when a massive star—one more than about ten times the sun’s mass—runs out of nuclear fuel, and its iron core collapses, creating a black hole or neutron star.

Between these two main types of supernovae are electron-capture supernovae.

Electron-capture supernovae have been theorized for four decades, but their real-world examples remained elusive.

For the first time, scientists from UC Santa Barbara at Las Cumbres Observatory have discovered the first convincing evidence for this new type of supernova. The discovery sheds light on the thousand-year mystery of the supernova from A.D. 1054 visible worldwide in the daytime before eventually becoming the Crab Nebula.

Electron-capture supernovae are believed to be originated from the explosions of massive super-asymptotic giant branch (SAGB) stars. In this supernova, some of the electrons in the oxygen-neon-magnesium core get crushed into their atomic nuclei in an electron capture interaction. This removal of electrons makes the core of the star clasp under its weight and collapse, resulting in an electron-capture supernova.

Scientists, in this study, used several telescopes around and above the globe. They discovered many unusual characteristics of supernova SN 2018zd. Some of these characteristics were observed for the first time in a supernova.

It helped that the supernova was somewhat close by—just 31 million light-years away—in the universe NGC 2146. This permitted the group to look at archival images taken by the Hubble Space Telescope preceding the explosion and to distinguish the likely progenitor star before it exploded. 

The observations were steady with another recently identified SAGB star in the Milky Way, yet conflicting with models of red supergiants, the progenitors of normal iron core-collapse supernovae.

Scientists observed all published data on supernovae. They found that some had a few of the indicators predicted for electron-capture supernovae. Only SN 2018zd had all six: an apparent SAGB progenitor, strong pre-supernova mass loss, an unusual stellar chemical composition, a weak explosion, little radioactivity, and a neutron-rich core.

Ken Nomoto at the Kavli IPMU of the University of Tokyo expressed excitement that his theory had been confirmed. “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. I very much appreciate the great efforts involved in obtaining these observations. This is a great case of the combination of observations and theory.”

Daichi Hiramatsu, a graduate student at UC Santa Barbara and Las Cumbres Observatory (LCO), said, “It was such a ‘Eureka moment’ for all of us that we can contribute to closing the 40-year-old theoretical loop, and for me, because my career in astronomy started when I looked at the stunning pictures of the Universe in the high school library, one of which was the iconic Crab Nebula taken by the Hubble Space Telescope.”

Andrew Howell, a staff scientist at Las Cumbres Observatory and adjunct faculty at UCSB, said, “The term Rosetta Stone is used too often as an analogy when we find a new astrophysical object, but in this case, I think it is fitting. This supernova is helping us decode thousand-year-old records from cultures all over the world. And it is helping us associate one thing we don’t fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova. In the process, it is teaching us about fundamental physics: how some neutron stars get made, how extreme stars live and die, and about how the elements we’re made of getting created and scattered around the universe.”

Journal Reference:
  1. Hiramatsu, D., Howell, D.A., Van Dyk, S.D. et al. The electron-capture origin of supernova 2018zd. Nat Astron (2021). DOI: 10.1038/s41550-021-01384-2

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