Darren.Osborne at csiro.au Darren.Osborne at csiro.au
Thu May 4 15:53:32 EST 2006

Issued by Helen Sim, Anglo-Australian Observatory (Sydney, Australia)

+61-(0)2-9372-4251 (office)

+61-(0)419-635-905 (mob.)

hsim at aaoepp.aao.gov.au



Image links and full media release http://www.gemini.edu/2001igpr



4 May 2006                                

For immediate use






Australian astronomers have explained why a dying star sent out mixed 

signals about its identity.


A cosmic explosion seen in 2001 looked like one particular kind of 

dying star, but then mysteriously seemed to change into the 

death-throes of another kind.


Dr Stuart Ryder of the Anglo-Australian Observatory and his colleagues 

now say this odd behaviour was caused by a 'companion' star that still 

lurks at the scene of the explosion.


Called supernova 2001ig, the explosion took place 37 million 

light-years away in a galaxy called NGC 7424, and was first spotted in 

2001 by a renowned Australian amateur supernova hunter, Bob Evans.


After studying the explosion, Ryder and his colleagues predicted the 

existence of the companion star. They recently found it, using the 

Gemini South telescope in Chile.


This is only the second time a companion has been found where a star 

has exploded.


But Ryder thinks companion stars could explain much that has puzzled 



"We've been putting supernovae into different categories for decades, 

but it's been a mystery how some of the different types are related to 

each other, and what's actually going on physically."


"In this case, the companion star and the one that exploded would have 

been orbiting each other. We think the companion helped strip hydrogen 

gas from the other star before it exploded," he said.


"There was a trace of hydrogen in the supernova early on, but it soon 

disappeared. So at first it looked like we had one kind of star 

exploding, but later on it looked quite different."


Two lines of evidence led the astronomers to suspect a companion was at 



This supernova (called SN2001ig) was similar to another rare one found 

in 1993 (called SN1993J). Astronomers found a companion at the site of 

the 1993 supernova, suggesting that this supernova might have one too.


And observations made with a CSIRO radio telescope near Narrabri, NSW, 

showed that there were thick patches of gas and dust in the space 

around the star.


"That gas and dust was shed by the star before it died," explained 

Ryder. "We think the orbiting companion swept it into a spiral shape - 

the bumps and dips in our radio data fit such a spiral. And people have 

seen these spirals around other stars with companions."




Dr Stuart Ryder, Anglo-Australian Observatory (Sydney)

+61-(0)2-9372-4843 (office)

+61-(0)419-970-834 (mob.)

sdr at aaoepp.aao.gov.au




A paper on the observations, "A post-mortem investigation of the Type 

IIb supernova 2001ig", co-authored by Ryder, University of Tasmania 

graduate student Clair Murrowood and former AAO astronomer Dr Raylee 

Stathakis, was published online in Monthly Notices of the Royal 

Astronomical Society on May 2. It is also available at 

http://au.arxiv.org/abs/astro-ph/0603336 .




Galaxy NGC 7424 and the supernova field http://www.gemini.edu/2001igpr


Spirals of gas and dust around Wolf-Rayet stars, imaged by Dr Peter 

Tuthill, University of Sydney





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