591 high velocity stars in the Galactic halo selected from LAMOST DR7
  and Gaia DR2
  In this paper, we report 591 high velocity star candidates (HiVelSCs)
selected from over 10 million spectra of the data release seven (DR7) of the
Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia
data release, with three-dimensional velocities in the Galactic rest-frame
larger than 445 km/s. We show that at least 43 HiVelSCs are unbound to the
Galaxy with escape probabilities larger than 50%, and this number decreases to
eight if the possible parallax zero-point error is corrected. Most of these
HiVelSCs are metal-poor and slightly alpha-enhanced inner halo stars. Only 14%
of them have [Fe/H] > -1, which may be the metal-rich "in situ" stars in halo
formed in the initial collapse of the Milky Way or metal-rich stars formed in
the disk or bulge but kinematically heated. The low ratio of 14% implies that
the bulk of stellar halo was formed from the accretion and tidal disruption of
satellite galaxies. In addition, HiVelSCs on the retrograde orbits have a
slightly lower metallicities on average compared with these on the prograde
orbits, meanwhile metal-poor HiVelSCs with [Fe/H] < -1 have an even faster mean
retrograde velocity compared with metal-rich HiVelSCs. To investigate the
origins of HiVelSCs, we perform orbit integrations and divide them into four
types, i.e., hypervelocity stars, hyper-runaway stars, runaway stars and fast
halo stars. A catalog for these 591 HiVelSCs, including radial velocities,
atmospheric parameters, Gaia astrometric parameters, spatial positions, and
velocities, etc., is available at
http://paperdata.chinavo.org/LYB/lamostdr7_gaiadr2_hvs_591.csv.
