MMT Hypervelocity Star Survey. II. Five New Unbound Stars
  We present the discovery of five new unbound hypervelocity stars (HVSs) in
the outer Milky Way halo. Using a conservative estimate of Galactic escape
velocity, our targeted spectroscopic survey has now identified 16 unbound HVSs
as well as a comparable number of HVSs ejected on bound trajectories. A
Galactic center origin for the HVSs is supported by their unbound velocities,
the observed number of unbound stars, their stellar nature, their ejection time
distribution, and their Galactic latitude and longitude distribution. Other
proposed origins for the unbound HVSs, such as runaway ejections from the disk
or dwarf galaxy tidal debris, cannot be reconciled with the observations. An
intriguing result is the spatial anisotropy of HVSs on the sky, which possibly
reflects an anisotropic potential in the central 10-100 pc region of the
Galaxy. Further progress requires measurement of the spatial distribution of
HVSs over the southern sky. Our survey also identifies seven B supergiants
associated with known star-forming galaxies; the absence of B supergiants
elsewhere in the survey implies there are no new star-forming galaxies in our
survey footprint to a depth of 1-2 Mpc.
