Discovery of two new hypervelocity stars from the LAMOST spectroscopic
  surveys
  We report the discovery of two new unbound hypervelocity stars (HVSs) from
the LAMOST spectroscopic surveys. They are respectively a B2V type star of ~ 7
M$_{\rm \odot}$ with a Galactic rest-frame radial velocity of 502 km/s at a
Galactocentric radius of ~ 21 kpc and a B7V type star of ~ 4 M$_{\rm \odot}$
with a Galactic rest-frame radial velocity of 408 km/s at a Galactocentric
radius of ~ 30 kpc. The origins of the two HVSs are not clear given their
currently poorly measured proper motions. However, the future data releases of
Gaia should provide proper motion measurements accurate enough to solve this
problem. The ongoing LAMOST spectroscopic surveys are expected to yield more
HVSs to form a statistical sample, providing vital constraint on understanding
the nature of HVSs and their ejection mechanisms.
