Rare 'young-looking' supernova found in old galaxy

Using the Pan-STARRS telescope on Haleakala, astronomers have found a young-looking supernova on the outskirts of a bright elliptical galaxy. Supernova PS1-12sk is rare in more ways than one.

"This supernova is one-of-a-kind. And it's definitely in the wrong neighborhood," said Nathan Sanders of the Harvard-Smithsonian Center for Astrophysics (CfA), lead author of the discovery paper.

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

Although the origin of this supernova type is unclear, the most likely cause seems to be the explosion of a massive star that previously ejected massive amounts of helium gas, much like Eta Carinae's Homunculus Nebula.

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. (Since massive stars don't live long, they don't stray far from where they are born before exploding).

PS1-12sk is different. It was found in an old galaxy located about 780 million light-years from Earth. 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 favours the prepared," said second author Alicia Soderberg of the CfA.

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.