Mysterious supernova may have carbon origins

BY KEITH COOPER

ASTRONOMY NOW

Posted: 2 June, 2009

A weird explosion in space that confounded astronomers may have been the destruction of a rare star with an unusual amount of carbon dust surrounding it, according to research carried out by scientists at the University of Warwick.

The explosion, which was first spotted on 21 February 2006 during a survey of a galaxy cluster in the constellation of Boötes by the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory, has perplexed astronomers. Normal supernovae usually take around three weeks to reach peak brightness; this object, designated SCP 06F6, took 100 days to rise to magnitude +21, before taking another hundred days to fade away. Normal supernovae tend to have more asymmetric light curves than that. Furthermore, the European Space Agency’s XMM-Newton spacecraft spotted a bright X-ray glow coming from SCP 06F6, and yet no host galaxy has ever been identified. Even the distance to SCP 06F6 has been uncertain, because astronomers have been unable to decode the spectrum of the explosion, until now.

Before and after comparison pictures of SCP 06F6 taken by the Hubble Space Telescope.

Image: NASA/ESA/K Barbary (University of California, Berkeley).

A team led by Dr Boris Gaensicke and including Dr Andrew Levan, Professor Thomas Marsh and Dr Peter Wheatley, all from Warwick, think that whatever exploded was living in a thick shell of carbon dust, about two billion light years away. This would match up with the redshifted lines in the spectrum. “The optical light emitted at peak brightness came from a cool carbon-rich atmosphere, resembling the spectrum of a carbon star,” Dr Gaensicke told Astronomy Now. However, it couldn’t have been a fully-fledged carbon star, as typical carbon stars are not massive enough to explode as supernovae. Rather, Gaensicke’s team think that it may have been a massive star that went supernova inside a thick cloud of carbon-rich dust, similar to the environments found around some Wolf–Rayet stars.

“A thick circumstellar layer would explain the long time scale of SCP 06F6, and could help explain the large X-ray luminosity,” says Dr Gaensicke.

A number of other ideas to try and explain SCP 06F6 have been put forward over the last three years, including asteroids or even a white dwarf being destroyed and swallowed by a black hole. The lack of any visible host galaxy would suggest that it is extremely faint and without an active black hole at its centre, which would seem to rule those alternative theories out.
“Several new telescopes are now being designed and built that will continuously monitor the entire sky for short guest appearances of new stars,” says Gaensicke, whose research is published in the 1 June issue of Astrophysical Journal Letters. “There is no doubt that SCP 06F6 will not remain alone in puzzling astronomers over the coming years.”