OLD-POPULATION HYPERVELOCITY STARS FROM THE GALACTIC CENTER: LIMITS FROM THE SLOAN DIGITAL SKY SURVEY

We present limits on the ejection of solar-metallicity (metal-rich) old-population hypervelocity stars (HVS) from a sample of over 290,000 stars selected from the Sloan Digital Sky Survey. We derive the speed at the solar circle from the measured positions and radial velocities (RVs) by assuming a radial orbit and adopting a simple isothermal model of the Galactic halo, which enables us to identify candidate bound and unbound ejectees. We examine the kinematics and metallicity distribution of this sample and find no metal-rich ejectees, from which our limits are derived. However, while tuned for metal-rich stars, our experiment is also sensitive to metal-poor ejectees. We find four candidate bound metal-poor F-stars from this sample, all with negative Galactocentric RV (i.e., returning toward the Galactic center (GC)). We additionally find two candidate metal-poor unbound stars (one F and one G). However, existing proper-motion measurements of these two stars make them unlikely to be emerging from the GC. The metal-rich nondetection places a limit on the rate of ejection of old-population stars from the GC of <35 Myr-1. Comparing to the rate for more massive B-star ejectees of ∼0.6 Myr-1, our limit on the rate of ejection of old-population HVS shows that the mass function at the GC is not bottom-heavy and is consistent with being normal to top-heavy. Future targeted surveys of old-population HVS could determine if it is indeed top-heavy.