Cosmic Explosion as a Star Sling

Black holes are not the only starting point for so-called hyper-speedboats

The young star, now in the galactic halo, was thrown from the outer edge of the galactic disk and not, as expected, from the galactic center (schematic exterior view of the Milky Way). © MPA and ESO
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Normally, a star can not escape the massive attraction of a galaxy. All the more surprising in 2005 was the discovery of so-called hyper-skiers. Only the black hole in the center of the Milky Way seemed to be an energy source. Because black holes can not only bind matter irrevocably, but also hurl strong away from itself. Now astronomers have discovered an object on the edge of the Milky Way that does not support this assumption.

The star, which is inexorably heading for the intergalactic space, is instead the remaining partner of a binary star system that has been torn apart by an explosion, the researchers said.

Unusual hyper-skater

The star was noticed by the research team led by Professor Ulrich Heber, Norbert Przybilla (both University of Erlangen-Nuremberg) and Fernanda Nieva (Max Planck Institute for Astrophysics) in the analysis of data from the 2.2 meter telescope of the European Southern Observatory (ESO). HD 271791 is eleven times as heavy as the sun and speeds through space at 2.2 million kilometers per hour. This puts him among the hyper-runners.

But this star is unlike its super fast comrades. The calculation of his trajectory showed that the four million solar black hole in the center of the Milky Way could not have been his starting point. "On the contrary, " explains Heber, "the star must come from the outer regions of the galactic disk, where there are no massive black holes at all."

More precise indications of the origin were finally provided by the use of a new analytical technique that precisely determines the chemical elements on the surface of the star. The composition differs from the normal stars. In excess are elements that are produced in supernovae, such as silicon. The surface of the star was so contaminated according to the findings of the researchers by material that has ejected a supernova explosion in his vicinity. display

Tight orbit

The team of scientists therefore concludes that HD 271791 originally had an even more massive companion that exploded after only a few million years of his life. His partner was catapulted out into space and flew straight on at line speed. Such a process would be as powerful as a black hole slingshot.

So far, however, experts have excluded such a scenario for the emergence of hypercancers. The orbit has to be very narrow, especially for such an extremely high speed. The original companion was allowed to be only a few sun radii large before it exploded. Since particularly massive stars can transform into a more compact variant before the explosion, the research group has proposed a model in which HD 271791 originally revolved around a star of at least 55 times the mass of the Sun.

Spectacular burst of energy

Immediately before the collapse of this star, the orbital period was only one day. The explosion - an exceptionally spectacular burst of energy, as Heber notes - made the heavier partner of the binary star system shrink to a black hole and set free the other whose previous orbital speed was now sufficient to leave the galaxy.

In the fall of 2005, Heber and his associates had already announced the discovery of another superfast star. At that time it was unclear whether the celestial object came from the center of the Milky Way or - which was also surprising - from the Great Magellanic Cloud, our neighbor's galaxy.

(idw - University of Erlangen-N rnberg, 04.03.2009 - DLO)