January 10, 2014
New Class Of Hypervelocity Stars Can ‘Escape’ From Their Galaxy
A surprising new class of hypervelocity stars has been discovered by an international team of astronomers. These solitary stars are moving fast enough to escape the gravitational grasp of the Milky Way galaxy, according to a study presented at the annual meeting of the American Astronomical Society. The findings were also published in the Astrophysical Journal.
"These new hypervelocity stars are very different from the ones that have been discovered previously," said Vanderbilt University graduate student Lauren Palladino. "The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small – about the size of the sun – and the surprising part is that none of them appear to come from the galactic core."
Palladino made the new discovery while working with assistant professor of astronomy Kelly Holley-Bockelmann to map the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey—a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.
"It's very hard to kick a star out of the galaxy," said Holley-Bockelmann. "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 hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism."
According to calculations, a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity. Scientists estimate that the Milky Way’s central black hole has a mass equivalent to four million suns. This is large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.
An example of the typical process involves a pair of binary stars that get caught in the central black hole’s grip. One of the stars spirals in towards the black hole, while its companion is flung outward at a tremendous velocity. Eighteen giant blue hypervelocity stars that might have been produced by such a mechanism have been identified so far.
The research team intends to continue their studies with additional observations.
Because the new rogue stars appear to have the same composition as normal disk stars, the scientists do not think they were born in the galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the galaxy.
"The big question is: what boosted these stars up to such extreme velocities? We are working on that now," said Holley-Bockelmann.