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02 Oct 2007

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Science Users=20

ESO 31/07 - Science Release

12 July 2007
For immediate release

The Gobbling Dwarf that Exploded

VLT Provides Evidence for Type Ia Supernovae = Scenario

A unique set of observations, obtained with ESO's = VLT, has=20 allowed astronomers to find direct evidence for the material that = surrounded a=20 star before it exploded as a Type Ia supernova. This strongly supports = the=20 scenario in which the explosion occurred in a system where a white dwarf = is fed=20 by a red giant.

=20
ESO PR Photo 31a/07
Evolution of SN 2006X Spectrum =

Because Type Ia supernovae are extremely luminous and = quite=20 similar to one another, these exploding events have been used = extensively as=20 cosmological reference beacons to trace the expansion of the Universe. =

However, despite significant recent progress, the = nature of the=20 stars that explode and the physics that governs these powerful = explosions have=20 remained very poorly understood.

In the most widely accepted models of Type Ia = supernovae the=20 pre-explosion white dwarf star orbits another star. Due to the close = interaction=20 and the strong attraction produced by the very compact object, the = companion=20 star continuously loses mass, 'feeding' the white dwarf. When the mass = of the=20 white dwarf exceeds a critical value, it explodes.

The team of astronomers studied in great detail SN = 2006X, a Type=20 Ia supernova that exploded 70 million light-years away from us, in the = splendid=20 spiral Galaxy Messier 100 (see ESO 08/06). Their observations led them = to=20 discover the signatures of matter lost by the normal star, some of which = is=20 transferred to the white dwarf.

The observations were made with the Ultraviolet and = Visual=20 Echelle Spectrograph (UVES), mounted at ESO's 8.2-m Very Large = Telescope, on=20 four different occasions, over a time span of four months. A fifth = observation=20 at a different time was secured with the Keck telescope in Hawaii. The=20 astronomers also made use of radio data obtained with NRAO's Very Large = Array as=20 well as images extracted from the NASA/ESA Hubble Space Telescope = archive.

=20
ESO PR Photo 31b/07
SN 2006X, before and after
the = Type Ia=20 Supernova explosion

"No Type Ia supernova has ever been observed at this = level of=20 detail for more than four months after the explosion," says Ferdinando = Patat,=20 lead author of the paper reporting the results in this week's issue of = Science=20 Express, the online version of the Science research journal. "Our data = set is=20 really unique."

The most remarkable findings are clear changes in the = absorption=20 of material, which has been ejected from the companion giant star. Such = changes=20 of interstellar material have never been observed before and demonstrate = the=20 effects a supernova explosion can have on its immediate environment. The = astronomers deduce from the observations the existence of several = gaseous shells=20 (or clumps) which are material ejected as stellar wind from the giant = star in=20 the recent past.

"The material we have uncovered probably lies in a = series of=20 shells having a radius of the order of 0.05 light-years, or roughly 3 = 000 times=20 the distance between Earth and the Sun", explains Patat. "The material = is moving=20 with a velocity of 50 km/s, implying that the material would have been = ejected=20 some 50 years before the explosion."

Such a velocity is typical for the winds of red = giants. The=20 system that exploded was thus most likely composed of a white dwarf that = acted=20 as a giant 'vacuum cleaner', drawing gas off its red giant companion. In = this=20 case however, the cannibal act proved fatal for the white dwarf. This is = the=20 first time that clear and direct evidence for material surrounding the = explosion=20 has been found.

"One crucial issue is whether what we have seen in SN = 2006X=20 represents the rule or is rather an exceptional case," wonders Patat. = "But given=20 that this supernova has shown no optical, UV and radio peculiarity = whatsoever,=20 we conclude that what we have witnessed for this object is a common = feature=20 among normal SN Ia. Nevertheless, only future observations will give us = answers=20 to the many new questions these observations have posed to us."

A high resolution image of SN 2006X in the spiral = galaxy Messier=20 100 is available as ESO=20 Press Photo 08a/06.

More Information

These results are reported in a paper in Science = Express=20 published on 12 July 2007 ("Detection of circumstellar material in a = normal Type=20 Ia Supernova", by F. Patat et al.).

The team is composed of F. Patat and L. Pasquini = (ESO), P.=20 Chandra and R. Chevalier (University of Virginia, USA), S. Justham, Ph.=20 Podsiadlowski , and C. Wolf (University of Oxford, UK), A. Gal-Yam and = J.D.=20 Simon (California Institute of Technology, Pasadena, USA), I.A. Crawford = (Birkbeck College London, UK), P.A. Mazzali, W. Hillebrandt, and N. = Elias-Rosa=20 (Max-Planck-Institute for Astrophysics, Garching, Germany), A.W.A. = Pauldrach=20 (Ludwig-Maximilians University, Munich, Germany), K. Nomoto (University = of=20 Tokyo, Japan), S. Benetti, E. Cappellaro, A. Renzini , F. Sabbadin, and = M.=20 Turatto (INAF-Osservatorio Astronomico, Padova, Italy), D.C. Leonard = (San Diego=20 State University, USA), and A. Pastorello (Queen's University Belfast, = UK). P.A.=20 Mazzali is also associated with INAF/Trieste, Italy.

Note

During Type Ia supernova events, remnants of stars = with an=20 initial mass of up to a few times that of the Sun (so-called "white = dwarf=20 stars") explode, leaving nothing behind but a rapidly expanding cloud of = "stardust". Their peak brightness rivals that of their parent galaxy, = hence=20 qualifying them as prime cosmic yardsticks. Type Ia supernovae are = apparently=20 quite similar to one another. This provides them a very useful role as = 'standard=20 candles' that can be used to measure cosmic distances. Astronomers have=20 exploited this fortunate circumstance to study the expansion history of = our=20 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=20 represent the end products of the evolution of solar-like stars. During = most of=20 their life, such stars draw most of their energy from the transformation = of=20 hydrogen into helium. But at some moment, the hydrogen fuel will run = out: this=20 phase - still many billions of years into the future for the Sun - = signals the=20 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=20 a planetary nebula. After the planetary nebula has dissipated into = interstellar=20 space, the star left behind is a white dwarf.

Contacts

Ferdinando Patat
ESO, Garching, Germany
Phone:=20 +49-89-3200-6744
Email: fpatat (at) eso.org

National contacts for the media:
Belgium - Dr. Rodrigo = Alvarez=20 +32-2-474 70 50 rodrigo.alvarez@oma.be
Czech Republic - Pavel Suchan = +420=20 267 103 040 suchan@astro.cz
Finland - Ms. Tiina Raivo +358 9 7748 = 8369=20 tiina.raivo@aka.fi
Denmark -=20 Dr. Michael Linden-V=C3=B8rnle +45-33-18 19 97 mykal@tycho.dk
France = - Dr. Daniel=20 Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude=20 +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio = +39-347-230 26=20 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 = 74 80=20 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833=20 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196=20 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54=20 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 = 82=20 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt=20 +44-1793-44 20 25 peter.barratt@stfc.ac.uk=20

Last Update: 25.07.07 =C2=A9ESO	Send=20 us your comments! \| Subscribe = to=20 Newsletter \| Privacy=20 Statement