Hypervelocity Stars in the Gaia era. Revisiting the most extreme stars
  from the MMT survey
  The hypervelocity star (HVS) survey conducted at the Multiple Mirror
Telescope (MMT) identified 42 B-type stars in the Galactic halo whose radial
velocity in the Galactic rest-frame exceeds $+275\,$km$\,$s${}^{-1}$. In order
to unravel the nature and origin of those high-velocity outliers, their
complete six-dimensional phase space information is needed. To this end, we
complemented positions and proper motions from the second data release of {\it
Gaia} with revised radial velocities and spectrophotometric distances that are
based on a reanalysis of the available MMT spectra of 40 objects using
state-of-the-art model spectra and a tailored analysis strategy. The resulting
position and velocity vectors for 37 stars were then used as input for a
subsequent kinematic investigation to obtain as complete a picture as possible.
The combination of projected rotational velocity, position in the Kiel diagram,
and kinematic properties suggests that all objects in the sample except two
(B576, B598) are very likely to be main sequence stars. While the available
data are still not precise enough to constrain the place of origin for 19
program stars, we identified eight objects that either come from the outer rim
of the Galactic disk or not from the disk at all, along with ten that
presumably stem from the Galactic disk. For almost all of those 18 targets with
more or less well-constrained spatial origin, the Galactic center (GC) is
disqualified as a possible place of origin. The most notable exception is B576,
the origin of which coincides extremely well with the GC when assuming a blue
horizontal branch (BHB) nature for it. HVS$\,$22 is by far the most extreme
object in the sample. Although its origin is completely unconstrained, an
ejection from the GC by the Hills mechanism is the most plausible explanation
for its current Galactic rest-frame velocity of
$1530^{+690}_{-560}\,$km$\,$s${}^{-1}$.
