The astronomers - including scientists at Stockholm University - interpret the stellar explosions as the outcome of a very massive star, probably more than 25 times heavier than our Sun, that lost a large fraction of its mass in strong winds before the explosion.
The stripped star collapses and explodes and as some of the gas is thrown out at high velocities, far above 10 000 kilometers per second, it collides with the material that the star previously lost. This is visible as a luminous supernova in the night sky. Unique in a Type Icn explosion is that no traces of hydrogen and helium – the elements that otherwise make up 99 percent of the universe – are found.
The discovery results from the dedicated efforts of the Zwicky Transient Facility project, a large camera mounted on an old telescope in California run by an international collaboration of astronomers, including the Stockholm team at the Oskar Klein Centre.
In a companion paper, a second object of this kind was followed in greater detail using the Hubble Space Telescope, corroborating the picture of an interaction-driven metal-rich explosion. Later observations, when the interaction had ceased, failed to detect the exploded star – suggesting that the core of the massive star imploded into an invisible black hole.
“This is a great discovery. With ZTF we find thousands of supernovae, and finding many of them also means we will get these extremely rare examples. The trick is not only to find all of them, but to recognize which ones are important to follow in detail,” explains researcher Steve Schulze from the Department of Physics at Stockholm University.
"The Nordic Optical Telescope (NOT) on La Palma was used when following up these two events," says Professor Jesper Sollerman at the Department of Astronomy at Stockholm University. “This telescope is partly run by Stockholm university and was fundamental to our understanding of this new type of supernova."
The papers are presented on 12 January 2022 at a press conference in conjunction with the American Astronomical Society meeting.
The paper A WC/WO star exploding within an expanding carbon-oxygen-neon nebula is published today in Nature and is led by Avishay Gal-Yam from The Weizmann Institute and describes the first Type Icn supernova SN 2019hgp. Co-authors from Stockholm University are Steve Schulze at the Department of Physics, and Jesper Sollerman, Erik Kool and Claes Fransson all at the Department of Astronomy.
The companion paper The Type Icn SN 2021csp: Implications for the Origins of the Fastest Supernovae and the Fates of Wolf-Rayet Stars in Astrophysical Journal is led by Dan Perley at Liverpool John Moores University and includes Jesper Sollerman, Steve Schulze, Erik Kool, Ragnhild Lunnan, and Ting-Wan Chen from Stockholm University.
