An international team of astronomers has announced the discovery of a very rare Type Ibn supernova on the outskirts of a bright elliptical galaxy located about 780 million light-years away.

“This supernova is one-of-a-kind. And it’s definitely in the wrong neighborhood,” said Dr Nathan Sanders of the Harvard-Smithsonian Center for Astrophysics, lead author of the discovery paper submitted for publication in the Astrophysical Journal (arXiv.org version).

Based on the presence of helium and other features, the object, dubbed PS1-12sk, is classified as a very rare Type Ibn supernova – only the sixth such example found out of thousands of supernovae.

A new subclass of ‘Type Ibn,’ characterized by intermediate-width He I emission, has emerged since the discovery of the supernova SN 1999cq. Although the origin of these supernovae is unclear, the most likely cause seems to be the explosion of a massive star that previously ejected massive amounts of helium gas. That origin was supported by the fact that the five previous Type Ibn supernovae were all found in galaxies like the Milky Way that are actively forming stars.

PS1-12sk is different. The site of the explosion shows no signs of recent star formation, and a supernova from a massive star has never before been seen in a galaxy of this type.

“It could be that we simply got very lucky with this discovery. But luck favors the prepared,” said co-author Dr Alicia Soderberg, also of the Harvard-Smithsonian Center for Astrophysics.

The finding suggests that the host galaxy might be hiding a star factory, allowing it to form massive stars where none were expected. Alternatively, PS1-12sk might have an entirely different origin such as a collision of two white dwarfs, one of which was helium-rich.

“Is this a runaway star from another star formation site? Is it a very local bit of star formation? Is it a different way for such a supernova to occur? None of these seems very likely so we have a real puzzle,” said co-author Dr John Tonry of the University of Hawaii Institute for Astronomy.

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Bibliographic information: Sanders NE et al. 2013. PS1-12sk is a Peculiar Supernova From a He-rich Progenitor System in a Brightest Cluster Galaxy Environment. Submitted for publication in ApJ; arXiv: 1303.1818