2007ApJ...664..343K

To date, all of the reported hypervelocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young. Old-population HVSs could be much more numerous than the young ones that have been discovered, but still have escaped detection because they are hidden in a much denser background of Galactic halo stars. Discovery of these stars would shed light on star formation at the Galactic center, would constrain the mechanism by which they are ejected from it, and, if they prove numerous, would enable detailed studies of the structure of the dark halo. We analyze the problem of finding these stars and show that the search should be concentrated around the main-sequence turnoff (0.3<g-i<1.1) at relatively faint magnitudes (19.5<g<21.5). If the ratio of turnoff stars to B stars is the same for HVSs as it is in the local disk, such a search would yield about 1 old-population HVS per 45 deg². A telescope similar to the Sloan 2.5 m could search about 20 deg² per night, implying that such a population, should it exist, would show up in interesting numbers in short order.