The swan song of super-massive stars lasts several hours
An international team making up the Figaro 1 cooperation (France - Italy - Australia) including astronomers from the ARTEMIS laboratory in Nice, IRAP in Toulouse and LAM in Marseille recently discovered that very massive stars could explode following an original way. This discovery is submitted today at the International Symposium on gamma-ray bursts 2 in Nashville, United States. It was recently published in the Astrophysical Journal.
Figure 1: Explosion of a gamma-ray burst: after the formation of a black hole at the center of the star, the outer layers rush. A portion of the material will escape as two jets launched at the speed of light or so (credit: CNRS / ARTEMIS - Celine Lavalade)
Until today, we only knew two types of gamma-ray bursts. The "short" bursts on the one hand, lasting less than two seconds, and probably associated with the catastrophic end of a binary system of neutrons stars 3 whose members are inexorably moving closer and closer under the effect of the radiation of the gravitationnelles waves 4. On the other hand, the "long" bursts last from a few seconds to a few minutes. They are associated with the end of life of a massive star, of the order of ten times the mass of the Sun ; the star burns very quickly its nuclear fuel, the hydrogen then the heavier elements, next a black hole forms at its center, while the outer layers are rushing towards the center (Figure 1).
On December 9, 2011 the NASA Swift satellite detected a very powerful but also very original burst, named GRB 111209a. Contrarily to "classical" gamma bursts, it lasted at least 7 hours. He was immediately followed by an armada of instruments recording at all wavelengths, including the telescope TAROT of the CNRS located at the European Southern Observatory in Chile and the X-ray observatory XMM-Newton of the European Space Agency.
Finding an explanation for this new type of gamma-ray burst required to imagine a mechanism to power the jet on a so long term. Researchers have proposed that another "monster" is invoked: a blue supergiant star, making 50 times the Sun's mass and composed almost entirely of hydrogen, implodes at the end of its life, producing the black hole at its center. In this case, however, the huge size of such a star - it would extend beyond the orbit of Jupiter - implies that the outer layers will take several hours to rush to the center, feeding the jet during this period (Figure 2).
One problem is that normally, the blue supergiant stars are surrounded by a very strong stellar wind that strips them of their outer layers. Only the heart of the exploded star thus remains, soon after their formation. To maintain the outer layers in place, the team proposed that the star is composed almost entirely of hydrogen and helium. However, since the beginning of the universe, several generations of stars have been produced, each one enriching a little more the interstellar medium with heavy elements such as oxygen and carbon during supernova explosions. GRB 111209a was produced at a relatively moderate distance, at a time when the universe should already be enriched in heavy elements, responsible for life on Earth. These facts seem, at first, mutually incompatible. However, we now know that there are still areas, increasingly rare, composed almost entirely of hydrogen in our galactic neighborhood.
If a long burst were to occur in our environment, it would have fatal consequences for life on Earth, as astronomers have long pointed. However, the enrichment of heavy elements in the Universe makes that GRBs are becoming increasingly rare, especially in galaxies like ours, filled with evolved stars populations. The understanding of these extreme events is important for the synthesis of stars and the mechanisms that can produce life, but also threaten it.
Video: GRB 101225A, better known a the Christmas GRB. Astronomers have determined that it resulted from the collapse and explosion of a supergiant star hundreds of times larger than the sun. Credit: NASA Goddard Space Flight Center Scientific Visualization Studio
Fast International GRB Afterglow Robotic Observations, international collaboration supported in France by the Centre National de la Recherche Scientifique (CNRS/PNHE), Programme National Hautes Energies, l’Instituto Nazionale di AstroFisica (Italiy), and l’Australian Research Council.
Gamma-ray bursts were discovered in the seventies as sudden bursts of gamma rays detected by U.S. military satellites whose purpose was the detection of explosions of nuclear bombs in the atmosphere. By the years 1990, we realized that they occurred at considerable distances where the effects of the expansion of the universe arise. A material jet up to very close to the speed of light is emitted, which is observed by astronomers as a powerful emission of gamma and X rays, the most energetic forms of light but also in the optical, the infrared and the radio domains.
A neutron star has the mass of our Sun, but within ten kilometers instead of 600 000 km.
These sources are considered as the best candidates for the detection of gravitational waves by the French-Italian VIRGO instrument (CNRS / INFN), currently being developed.
The Ultralong Gamma-Ray Burst 111209a : The Collapse of a Blue Super-Giant, B. Gendre et al., The Astrophysical Journal, 2013 March 20, Volume 766, page 30
Michel Boer, Laboratoire ARTEMIS (Observatoire de la Côte d'Azur/CNRS/Université de Nice Sophia Antipolis)
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Stéphane Basa, Laboratoire d'Astrophysique de Marseille (LAM)
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Alain Klotz, IRAP (CNRS/Univesrité Paul Sabatier-Toulouse III)
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