VLT Provides Evidence for Type Ia Supernovae Scenario (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:
Ferdinando Patat
ESO, Garching, Germany
Phone: +49-89-3200-6744

For immediate release: 12 July 2007

ESO Science Release 31/07

The Gobbling Dwarf that Exploded

VLT Provides Evidence for Type Ia Supernovae Scenario

A unique set of observations, obtained with ESO's VLT, has allowed
astronomers to find direct evidence for the material that surrounded a
star before it exploded as a Type Ia supernova. This strongly supports the
scenario in which the explosion occurred in a system where a white dwarf
is fed by a red giant.

Because Type Ia supernovae are extremely luminous and quite similar to one
another, these exploding events have been used extensively as cosmological
reference beacons to trace the expansion of the Universe.

However, despite significant recent progress, the nature of the stars that
explode and the physics that governs these powerful explosions have
remained very poorly understood.

In the most widely accepted models of Type Ia supernovae the pre-explosion
white dwarf star orbits another star. Due to the close interaction and the
strong attraction produced by the very compact object, the companion star
continuously loses mass, 'feeding' the white dwarf. When the mass of the
white dwarf exceeds a critical value, it explodes.

The team of astronomers studied in great detail SN 2006X, a Type Ia
supernova that exploded 70 million light-years away from us, in the
splendid spiral Galaxy Messier 100 (see ESO 08/06). Their observations led
them to discover the signatures of matter lost by the normal star, some of
which is transferred to the white dwarf.

The observations were made with the Ultraviolet and Visual Echelle
Spectrograph (UVES), mounted at ESO's 8.2-m Very Large Telescope, on four
different occasions, over a time span of four months. A fifth observation
at a different time was secured with the Keck telescope in Hawaii. The
astronomers also made use of radio data obtained with NRAO's Very Large
Array as well as images extracted from the NASA/ESA Hubble Space Telescope
archive.

"No Type Ia supernova has ever been observed at this level of detail for
more than four months after the explosion," says Ferdinando Patat, lead
author of the paper reporting the results in this week's issue of Science
Express, the online version of the Science research journal. "Our data set
is really unique."

The most remarkable findings are clear changes in the absorption of
material, which has been ejected from the companion giant star. Such
changes of interstellar material have never been observed before and
demonstrate the effects a supernova explosion can have on its immediate
environment. The astronomers deduce from the observations the existence of
several gaseous shells (or clumps) which are material ejected as stellar
wind from the giant star in the recent past.

"The material we have uncovered probably lies in a series of shells having
a radius of the order of 0.05 light-years, or roughly 3 000 times the
distance between Earth and the Sun", explains Patat. "The material is
moving with a velocity of 50 km/s, implying that the material would have
been ejected some 50 years before the explosion."

Such a velocity is typical for the winds of red giants. The system that
exploded was thus most likely composed of a white dwarf that acted as a
giant 'vacuum cleaner', drawing gas off its red giant companion. In this
case however, the cannibal act proved fatal for the white dwarf. This is
the first time that clear and direct evidence for material surrounding the
explosion has been found.

"One crucial issue is whether what we have seen in SN 2006X represents the
rule or is rather an exceptional case," wonders Patat. "But given that
this supernova has shown no optical, UV and radio peculiarity whatsoever,
we conclude that what we have witnessed for this object is a common
feature among normal SN Ia. Nevertheless, only future observations will
give us answers to the many new questions these observations have posed to
us."

A high resolution image of SN 2006X in the spiral galaxy Messier 100 is
available as ESO Press Photo 08a/06.

More Information:

These results are reported in a paper in Science Express published on 12
July 2007 ("Detection of circumstellar material in a normal Type Ia
Supernova", by F. Patat et al.).

The team is composed of F. Patat and L. Pasquini (ESO), P. Chandra and R.
Chevalier (University of Virginia, USA), S. Justham, Ph. Podsiadlowski ,
and C. Wolf (University of Oxford, UK), A. Gal-Yam and J.D. Simon
(California Institute of Technology, Pasadena, USA), I.A. Crawford
(Birkbeck College London, UK), P.A. Mazzali, W. Hillebrandt, and N.
Elias-Rosa (Max-Planck-Institute for Astrophysics, Garching, Germany),
A.W.A. Pauldrach (Ludwig-Maximilians University, Munich, Germany), K.
Nomoto (University of Tokyo, Japan), S. Benetti, E. Cappellaro, A. Renzini
, F. Sabbadin, and M. Turatto (INAF-Osservatorio Astronomico, Padova,
Italy), D.C. Leonard (San Diego State University, USA), and A. Pastorello
(Queen's University Belfast, UK). P.A. Mazzali is also associated with
INAF/Trieste, Italy.
Note

During Type Ia supernova events, remnants of stars with an initial mass of
up to a few times that of the Sun (so-called "white dwarf stars") explode,
leaving nothing behind but a rapidly expanding cloud of "stardust". Their
peak brightness rivals that of their parent galaxy, hence qualifying them
as prime cosmic yardsticks. Type Ia supernovae are apparently quite
similar to one another. This provides them a very useful role as 'standard
candles' that can be used to measure cosmic distances. Astronomers have
exploited this fortunate circumstance to study the expansion history of
our Universe, leading to the conclusion that the Universe is expanding at
an accelerating rate (see e.g. ESO PR 21/98).

White dwarfs are Earth-size, hot and extremely dense stars that represent
the end products of the evolution of solar-like stars. During most of
their life, such stars draw most of their energy from the transformation
of hydrogen into helium. But at some moment, the hydrogen fuel will run
out: this phase -- still many billions of years into the future for the
Sun -- signals the beginning of profound, increasingly rapid changes in
the star which will ultimately lead to its death. The star dramatically
increases in radius, becoming a red giant. Later, it will expel huge
quantities of gas and appear as a planetary nebula. After the planetary
nebula has dissipated into interstellar space, the star left behind is a
white dwarf.

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