2017ApJ...847L...9H

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_☉ with a Galactic rest-frame radial velocity of 502 km s^–1 at a Galactocentric radius of ∼21 kpc and a B7V-type star of ∼4 M_☉ with a Galactic rest-frame radial velocity of 408 km s^–1 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 constraints on understanding the nature of HVSs and their ejection mechanisms.