What would fling a star out of the galaxy at over 1 million miles per hour? Warren Brown (Harvard-Smithsonian Center for Astrophysics) and colleagues have some thoughts on that, based on their own and other studies in Europe that have so far identified five stellar exiles, a group now called ‘hypervelocity stars.’ “These stars literally are castaways,” says Brown. “They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space.”
Brown’s team went after galactic escapees in a targeted manner, using computer models that showed such stars would be forced into their current trajectories by interactions in the galactic core. The idea is this: a binary star swings too close to the black hole at the galaxy’s center. Its gravity tears the duo apart, in the process capturing one of the stars and ejecting the other one at high velocity.
Evidence exists not only in the exiled stars themselves but in the other half of the binary pairs that once contained them; the stars the exiles leave behind orbit the central black hole in just the kind of elongated, elliptical orbit that would be expected from the computer models. Brown’s team figures a star is ejected from the galaxy about once every 100,000 years, although our knowledge of the extreme conditions at the core still needs a great deal of refinement to understand the process.
And in a unique way, these curious objects are interstellar probes of their own. By studying them, we learn something about the structure of the Milky Way. Says Margaret Geller, co-author of the paper on this work, “During their lifetime, these stars travel across most of the Galaxy. If we could measure their motions across the sky, we could learn about the shape of the Milky Way and about the way the mysterious dark matter is distributed.”
The team’s study has been submitted to the Astrophysical Journal Letters as “A Successful Targeted Search for Hypervelocity Stars,” now available online at the arXiv site.