Using the Gemini South telescope in Chile, the team said the presence of the companion explains why the supernova, which started off looking like one kind of exploding star, seemed to change its identity after a few weeks.

Originally, the Gemini observations were intended only to be reconnaissance for later imaging with the Hubble Space Telescope, "but the Gemini data were so good we got our answer straight away," said lead investigator, Stuart Ryder of the Anglo-Australian Observatory.

Renowned Australian supernova hunter Bob Evans first spotted supernova 2001ig in December 2001 on the outskirts of a spiral galaxy named NGC 7424, about 37 million light-years away in the southern constellation Grus, or the Crane.

The team monitored the supernova over the next month with optical telescopes in Chile. Astronomers classify the events according to features in their optical spectra. SN2001ig initially showed the telltale signs of hydrogen, which had it tagged as a Type II supernova, but the hydrogen later disappeared, which put it into the Type I category.

The problem was, only a few supernovae - classified as Type IIb, to indicate their change of identity - have ever been observed, and only one, SN 1993J, was located closer than SN 2001ig.

Astronomers examining the characteristics of SN1993J developed a hypothesis: the supernova's progenitor had a companion star that had stripped material off the star before it exploded. If so, the progenitor would have retained only a little hydrogen - so little that it could disappear from the supernova spectrum within a few weeks.

In 2003, observations by the Hubble Space Telescope and one of the twin giant Keck telescopes in Hawai`i confirmed SN 1993J did indeed have a companion. So, Ryder and colleagues wondered if SN2001ig might have had a companion as well, particularly because radio observations also hinted at a companion.

Soon after astronomers discovered SN2001ig, Ryder and colleagues began monitoring it with the Commonwealth Scientific and Industrial Research Organisation's Australia Telescope Compact Array. They found that its radio emissions did not fall off smoothly over time. Instead, they showed regular bumps and dips. This suggested the material around the star that exploded - which must have been shed late in its life - was unusually lumpy.

Although the lumps might have represented matter periodically shed from the convulsing star, their spacing suggested another explanation: they were generated by a companion in an eccentric orbit.

The companion, as it orbited, would have swept material shed by the progenitor into a spiral pattern, with denser lumps at the point in the orbit called the periastron where the two stars approached most closely.

Peter Tuthill of the University of Sydney had imaged such spirals around hot massive stars called Wolf-Rayet stars. When he used both Keck telescopes to observe SN2001ig, he found the bumps in supernova's radio light-curve were spaced in a way consistent with the curvature of one of the earlier spirals he had imaged.

"Stellar evolution theory suggests that a Wolf-Rayet star with a massive companion could produce this unusual kind of supernova," Ryder said.

If the supernova progenitor had a companion, it might become visible when the supernova debris had cleared. So, the team used the Gemini Multi-Object Spectrograph camera on the 8-meter Gemini South telescope.

When the time came to observe, the "seeing conditions" (stability of the atmosphere) were excellent, so the team needed just an hour and a half of observing time to image the supernova field - and reveal a yellow-green point-like object at the location of the supernova explosion.

"We believe this is the companion," Ryder said. "It's too red to be a patch of ionized hydrogen, and too blue to be part of the supernova remnant itself."

The companion has a mass between 10 and 18 times larger than the Sun's. The astronomers hope to use GMOS again in coming months to get a spectrum of the companion, to refine this estimate.

This is only the second time a companion star to a Type IIb supernova has been imaged, and the first time the imaging has been done from the ground.

Binary companions could explain much of the diversity seen in supernovae, Ryder said. "We've been able to show the chameleon-like behavior of SN2001ig has a surprisingly simple explanation."

Related Links
Gemini Observatory
Supernova Research Paper
Anglo-Australian Observatory

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