Scientists led by Lauren Palladino and Dr Kelly Holley-Bockelmann from Vanderbilt University have identified a new class of hypervelocity stars (HVSs) in the Milky Way Galaxy.
Top and side views of the Milky Way Galaxy show the location of 4 of the new class of HVSs. The general directions from which the stars have come are shown by the colored bands. Image credit: Julie Turner, Vanderbilt University / NASA / ESO.
HVSs – solitary stars moving fast enough to escape the gravitational grasp of our Galaxy – were first proposed by Los Alamos National Laboratory scientist Dr Jack Hills in 1988. Their existence was confirmed 17 years later.
“The original hypervelocity stars are large blue stars and appear to have originated from the Galactic Center,” said Lauren Palladino, the first author on the study published in the Astrophysical Journal (arXiv.org).
HVSs of the new type are very different from the ones that have been discovered previously.
These stars are relatively small – about the size of the Sun – and the surprising part is that none of them appear to come from the Milky Way’s Galactic Center.
The discovery came as the researchers were mapping our Galaxy by calculating the orbits of Sun-like stars in the SDSS Survey.
“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our Galaxy. None of these HVSs come from the center, which implies that there is an unexpected new class of HVS, one with a different ejection mechanism,” Dr Holley-Bockelmann said.
Scientists calculate that these stars must move at speeds of more than a million miles per hour relative to the motion of the galaxy to reach escape velocity.
They also estimate that the supermassive black hole at the Milky Way’s Center has a mass equivalent to 4 million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.
The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity.
So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.
Now Dr Schneider and his colleagues identified an additional 20 Sun-sized stars that they characterize as HVS candidates.
The new HVSs appear to have the same composition as normal disk stars, so the scientists do not think that their birthplace was in the Galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the Milky Way.
“The big question is: what boosted these stars up to such extreme velocities? We are working on that now,” Dr Holley-Bockelmann said.
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Palladino LE et al. 2014. Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample. ApJ 780, 7; doi: 10.1088/0004-637X/780/1/7
