Astrophys. J. 821, L1–L19 (2016)

Hypervelocity stars can reach speeds of 770 km s−1 (with respect to our galactic rest frame), exceeding the escape velocity from the Milky Way. How stars reach those speeds is a matter of debate. One theory involves a binary star system that gets too close to the supermassive black hole at the centre of the Galaxy, which tears apart the binary, keeps (or swallows) one star and accelerates the other star outwards with a high velocity. But Péter Németh and co-workers have shown that the binary star system, PB 3877, moving just below escape velocity, challenges such models.

The authors used the Keck II telescope in Hawaii and the Very Large Telescope in Chile to discover that a known hypervelocity star is actually in a wide binary system with a much cooler star. A reconstruction of their orbit suggests that the stars were never near the galactic centre. All other acceleration mechanisms, such as supernova explosions, would disrupt the binary stars. Thus Németh et al. speculated that dark matter may be holding the stars together, or that they have come from another galaxy.