First supernova companion star found (Forwarded)

European Space Agency
Science News Release SNR 1-2004
Paris, France 7 January 2004

First supernova companion star found

A joint European/University of Hawaii team of astronomers has for the first time
observed a stellar 'survivor' to emerge from a double star system involving an
exploded supernova.

Supernovae are some of the most significant sources of chemical elements in the
Universe, and they are at the heart of our understanding of the evolution of
galaxies.

Supernovae are some of the most violent events in the Universe. For many years
astronomers have thought that they occur in either solitary massive stars (Type
II supernovae) or in a binary system where the companion star plays an important
role (Type I supernovae). However no one has been able to observe any such
companion star. It has even been speculated that the companion stars might not
survive the actual explosion ...

The second brightest supernova discovered in modern times, SN 1993J, was found
in the beautiful spiral galaxy M81 on 28 March 1993. From archival images of
this galaxy taken before the explosion, a red supergiant was identified as the
mother star in 1993 -- only the second time astronomers have actually seen the
progenitor of a supernova explosion (the first was SN 1987A, the supernova that
exploded in 1987 in our neighbouring galaxy, the Large Magellanic Cloud).

Initially rather ordinary, SN 1993J began to puzzle astronomers as its ejecta
seemed too rich in the chemical element helium and instead of fading normally it
showed a bizarre sharp increase in brightness. The astronomers realised that a
normal red supergiant alone could not have given rise to such a weird supernova.
It was suggested that the red supergiant orbited a companion star that had
shredded its outer layers just before the explosion.

Ten years after this cataclysmic event, a European/University of Hawaii team of
astronomers has now peered deep into the glowing remnants of SN 1993J using the
NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys (ACS) and the
giant Keck telescope on Mauna Kea in Hawaii. They have discovered a massive star
exactly at the position of the supernova that is the long sought companion to
the supernova progenitor.

This is the first supernova companion star ever to be detected and it represents
a triumph for the theoretical models. In addition, this observation allows a
detailed investigation of the stellar physics leading to supernova explosions.
It is now clear that during the last 250 years before the explosion 10 solar
masses of gas were torn violently from the red supergiant by its partner. By
observing the companion closely in the coming years it may even be possible to
detect a neutron star or black hole emerge from the remnants of the explosion
'in real time'.

Given the paucity of observations of supernova progenitor systems this result,
published in Nature on 8 January 2004, is likely to "be crucial to understanding
how very massive stars explode and why we see such peculiar supernovae"
according to first author Justyn R. Maund from the University of Cambridge, UK.

Stephen Smartt, also from the University of Cambridge, says: "Supernova
explosions are at the heart of our understanding of the evolution of galaxies
and the formation of chemical elements in the Universe. It is essential that we
know what type of stars produce them." For the last ten years astronomers have
believed that they could understand the very peculiar behaviour of 1993J by
invoking the existence of a binary companion star and now this picture has
proved correct.

According to Rolf Kudritzki from the University of Hawaii, "The combination of
the outstanding spatial resolution of Hubble and the huge light gathering power
of the Keck 10m telescope in Hawaii has made this fantastic discovery possible."

Supernovae occur when a star of more than about eight times the mass of the Sun
reaches the end of its nuclear fuel reserves and can no longer produce enough
energy to keep the star from collapsing under its own immense weight. The core
of the star collapses, and the outer layers are ejected in a fast-moving shock
wave. This huge energy release causes the visible supernova we see. While
astronomers are convinced that observations will match this theoretical model,
they are in the embarrassing position that they have confidently identified only
two stars that later exploded as supernovae -- the precursors of supernovae
1987A and 1993J.

There have been more than 2000 supernovae discovered in galaxies beyond the
Milky Way and there appear to be about eight distinct sub-classes. However
identifying which stars produce which flavours has proved incredibly difficult.
This team has now embarked on a parallel project with the Hubble Space Telescope
to image a large number of galaxies and then wait patiently for a supernova to
explode. Supernovae appear in spiral galaxies like M81 on average once every 100
years or so. The team, led by Stephen Smartt, hope to increase the numbers of
supernova progenitors known from 2 to 20 over the next five years.

Notes for editors

The team is composed of Stephen J. Smartt and Justyn R. Maund (University of
Cambridge, UK), Rolf. P. Kudritzki (University of Hawaii, USA), Philipp
Podsiadlowski (University of Oxford, UK) and Gerry F. Gilmore (University of
Cambridge, UK).

Animations of the discovery and general Hubble Space Telescope background
footage are available from
http://www.spacetelescope.org/video/heic0401_vnr.html

For more information, please contact:

Justyn R. Maund
University of Cambridge
Tel: +44 (0)1223 337544
E-mail:

Stephen Smartt
University of Cambridge
Tel: +44 (0)1223 766 651
E-mail:

Rolf. P. Kudritzki
Institute for Astronomy
University of Hawaii
Tel: +1 808 956 8566
E-mail:

Lars Lindberg Christensen
Hubble European Space Agency Information Centre
Garching, Germany
Tel: +49 89 3200 6306 (089 within Germany)
E-mail:

[NOTE: Images supporting this release are available at
http://sci.esa.int/science-e/www/obj...objectid=34455 ]