Companion Explains "Chameleon" Supernova (Forwarded)

Gemini Observatory
Hilo Hawaii

Contacts:

Helen Sim
Anglo-Australian Observatory (Sydney Australia)
+61(2)9372-4251(Desk)

Dr. Stuart Ryder
Anglo-Australian Observatory
+61(2)9372-4843 (Desk)

Peter Michaud
Gemini Observatory, Hilo HI
(808) 974-2510 (Desk)

For Release on Wednesday, May 3rd, 2006

Companion Explains "Chameleon" Supernova

Using the Gemini South telescope in Chile, Australian astronomers have
found a predicted "companion" star left behind when its partner exploded
as a very unusual supernova. 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.

The Gemini observations were originally intended 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,
Dr. Stuart Ryder of the Anglo-Australian Observatory (AAO).

Renowned Australian supernova hunter Bob Evans first spotted supernova
2001ig in December 2001. It lies in the outskirts of a spiral galaxy NGC
7424, which is about 37 million light-years away in the southern
constellation of Grus (the Crane).

The supernova was monitored over the next month by optical telescopes in
Chile. Supernovae are classified according to the 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.

But how could a supernova change its type? Only a handful of such
supernovae, classified as "Type IIb" to indicate their curious change of
identity, have ever been seen. Only one (called SN 1993J) was closer
than SN 2001ig.

Astronomers studying SN1993J had suggested an explanation: the
supernova’s progenitor had a companion star that stripped material off
the star before it exploded. This would leave only a little hydrogen on
the progenitor -- so little that it could disappear from the supernova
spectrum within a few weeks.

A decade later observations with the orbiting Hubble Space Telescope and
one of the Keck telescopes in Hawai’i confirmed that SN 1993J did indeed
have a companion. Ryder and colleagues wondered if SN2001ig might have
had a companion as well.

Radio Light Curve Shows Lumps & Bumps

Radio observations also hinted at a companion.

Soon after SN2001ig was discovered, Ryder and his colleagues began
monitoring it with a radio telescope, the CSIRO (Commonwealth Scientific
and Industrial Research Organisation) Australia Telescope Compact Array
in eastern Australia. The radio emission did not fall off smoothly over
time but instead showed regular bumps and dips. This suggested that the
material in space 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 was such that another explanation
seemed more likely: that they were generated by a companion in an
eccentric orbit. As it orbited, the companion would have swept material
shed by the progenitor into a spiral (pinwheel) pattern, with denser
lumps at the point in the orbit -- periastron -- where the two stars
approached most closely.

Such spirals have been imaged around hot, massive stars called
Wolf-Rayet stars by Dr Peter Tuthill of the University of Sydney, using
the Keck telescopes. The bumps in the radio light-curve of SN2001ig were
spaced in a way consistent with the curvature of one of the spirals
Tuthill has imaged.

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

If the supernova progenitor had a companion, it might be visible when
the supernova debris had cleared. So the astronomers put in a request to
observe with the GMOS (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. Just an hour and a half was needed 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," said Ryder. "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 of between 10 and 18 times that of the Sun. The
astronomers hope to use GMOS again in coming months to get a spectrum of
the companion, to refine this estimate.

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

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.

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

Gemini is an international partnership managed by the Association of
Universities for Research in Astronomy under a cooperative agreement
with the National Science Foundation.

The Gemini Observatory provides the astronomical communities in each
partner country with state-of-the-art astronomical facilities that
allocate observing time in proportion to each country's contribution. In
addition to financial support, each country also contributes significant
scientific and technical resources. The national research agencies that
form the Gemini partnership include: the US National Science Foundation
(NSF), the UK Particle Physics and Astronomy Research Council (PPARC),
the Canadian National Research Council (NRC), the Chilean Comisión
Nacional de Investigación Cientifica y Tecnológica (CONICYT), the
Australian Research Council (ARC), the Argentinean Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET) and the Brazilian
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
The Observatory is managed by the Association of Universities for
Research in Astronomy, Inc. (AURA) under a cooperative agreement with
the NSF. The NSF also serves as the executive agency for the
international partnership.

IMAGE CAPTION:
[http://www.gemini.edu/images/stories...2/fig1_med.jpg
(283KB)]
The Galaxy NGC 7424 as imaged with the Gemini South Mulit-object
Spectrograph. Inset shows field of SN2001ig as indicated by arrow.