New kind of supernova explosion reveals a naked star

Artist’s illustration of the aftermath of the first-of-its-kind supernova SN 2021yfj. Well before the explosion, the progenitor star had lost its outer layers, consisting only of its oxygen and silicon core – a star for which no analogue exists in the Milky Way. Yet, this stripped-to-the-bone star underwent further intense mass-loss episodes, ejecting shells of material rich in silicon (grey), sulphur (yellow), and argon (purple). The violent collision of these massive shells produced a brilliant supernova, visible even from 2 billion light-years away. This supernova is so unusual that it defines a new class, Type Ien. Credit: Keck Observatory/Adam Makarenko.

Exploding stars – supernovae – suddenly appear on the night sky and briefly shine for a few weeks. Current supernova telescopes, like the Zwicky Transient Facility (ZTF), find thousands of these each year and occasionally discover some very rare explosions. One of these explosions is supernova SN 2021yfj which appeared in the constellation Cetus in September 2021, located at a distance of about 2 billion light years from the Earth.

“This is the first time we have seen a star that was essentially stripped to the bone. It shows us how stars are structured and proves that stars can lose a lot of material before they explode,” says Steve Schulze who led the study. He was a postdoc at the Oskar Klein centre in Stockholm at the time the supernova was discovered and the investigation initiated.

Jesper Sollerman, professor at the Department of Astronomy at Stockholm University, is one of the co-investigators of the study:

“Most of the universe is made up of hydrogen and helium, but we have previously found exploding stars that are stripped of these elements and instead contain mostly carbon, oxygen and nitrogen. However, SN 2021yfj contains instead the even rarer elements silicon and sulphur. These are known to be produced in the very innermost cores of massive stars, but we have never before seen the evidence of this in such a clear way.”

During almost two months the international collaboration collected follow-up observations with several of the largest telescope available, including Keck on Hawaii, the ESO Very Large Telescope in Chile and the Nordic Optical Telescope on La Palma. Analysing these observations revealed emission from material surrounding the exploding star that was rich primarily in sulphur and silicon – in contrast to all previously studied supernova explosions.

The study

The paper Extremely-stripped supernova reveals a silicon and sulphur formation site is published in Nature on August 20. The study was led by Steve Schulze, former postdoc at the Oskar Klein Centre at Stockholm University and now based at Northwestern University, USA. From Stockholm University participated in addition to Steve Schulze and Jesper Sollerman also Ragnhild Lunnan, Nikhil Sarin, Seán Brennan, Claes Fransson, Anjasha Gangopadhyay and Anders Jerkstrand. 

The study can also be found on arxiv.org an open-access repository of non-peer reviewed electronic preprints and postprints.