A cosmological billiard : a new explanation for gamma ray bursts
1er avril 2006
Gamma ray bursts are the most powerful events in the Universe. They are commonly believed to result from the death of a massive star. By scrutinizing the residuals of such an explosion, a French Swiss team has discovered that gamma ray bursts occur several 1000s of light years away from regions containing large amounts of massive stars. To explain this, they have assumed that the GRB progenitor star had travelled through the galactic medium for few millions years after being expelled from a dense stellar cluster, like a ball ejected from a billiards table.
The origin of the gamma ray bursts is one of the most challenging questions of the 21st century. Although proven to be related to cosmological events, they are so powerful that, until 1973, they were formerly interpreted as resulting from nuclear bomb test on earth !
GRB-f1.jpg Figure 1 : Hubble Space Telescope image (made in 2001) of the ESO-184-G82 galaxy where a gamma ray burst occurs in 1998. The picture shows the location of the residual of the associated supernova that exploded 3000 light-years away from the massive star region. Click on the image to enlarge it
Two scenarios for an unique explosion
A gamma ray burst phenomenon indicate the death of a star which has mass from 30 to 100 times that of our Sun. This death is accompanied by the star collapse, and the emission of a considerable energy. This is a rare and spectacular event since it occurs approximately for only 1 massive star among 10000. One scenario assumes an extremely fast rotating star losing its envelope and then collapsing with the emission of a high energetic short burst. A second scenario supports a fusion between a star and its companion to be responsible of that burst. These two scenarios are still debated. They are however supported by the recent discovery of supernova which seem to be systematically associated to the gamma ray bursts.
Gamma ray bursts are always offset from massive star regions
By scrutinizing the residuals of a 1998 gamma ray burst in the galaxy ESO-184-G82, the French Swiss team has noticed the nearby presence of very numerous massive stars in the host galaxy. These stars, namely Wolf-Rayet stars, or WR (from the names of two astroomers from Paris Observatory), are well known to be often fast rotating and generally end their life in a supernova explosion. They have been often considered as being ideal to be progenitors of gamma ray bursts. Surprisingly, the gamma ray burst had occurred 3000 light-years off the compact star clusters including the massive stars (Figure 1). This unexpected offset has been confirmed by examination of other cases for which the team had images of sufficient quality. How can this be understood ?
Strong interactions between massive stars
The French Swiss team has found a natural explanation for explaining those puzzling properties. Massive stars are subjected to strong interactions within the core of massive stellar super clusters. Some of them can be kicked out after such events, such as a ball in a billiards game. Then having acquired large angular momentum the runaway star travel for few millions years, lose its envelope before exploding as a supernova and with a gamma ray burst. Its rapid rotation favors an emission of energy (and gamma rays) strongly dissymmetric.
Solving the enigma of gamma ray burst
Such unusual encountering between massive stars would easily account for the rarity of massive stars that end their life in a gamma ray burst. This brings a new contribution to the understanding of the origin of gamma ray bursts, which are among the most fascinating and brightest sources of the Universe.
(1) The team consists of François Hammer, Hector Flores, Emeric Le Fl’och and Mathieu Puech for l’Observatoire de Paris (GEPI) and Daniel Schaerer and Miroslava Dessauges for l’Observatoire de l’Université de Genève See also the Press Release de l’Observatoire de Genève
Reference Article 2006, Astronomy & Astrophysics in press "Detection of Wolf-Rayet stars in host galaxies of Gamma-ray bursts (GRBs) : are GRBs produced by runaway massive stars ejected from high stellar density regions ?" F. Hammer, H. Flores, D. Schaerer, M : Dessauges-Zawadsky, E : Le Fl’och, M. Puech
Contact François Hammer (Observatoire de Paris, GEPI) Hector Flores (Observatoire de Paris, GEPI)
Dernière modification le 4 mars 2013