A New Hyper-runaway Star Discovered from LAMOST and Gaia: Ejected Almost in the Galactic Rotation Direction

In this paper, we report the discovery of a new late-B-type unbound hyper-runaway star( LAMOST-HVS4) from the LAMOSTspectroscopic surveys. According to its atmospheric parameters, it is either a B-type main-sequence(MS) staror a blue horizontal branch (BHB) star. Its Galactocentric distance and velocity are 30.3 +/- 1.6 kpc and 586 +/- 7 km s(-1) if it is an MS star, and they are 13.2 +/- 3.7 kpc and 590 +/- 7 km s(-1) if a BHB star. We track its trajectories back and find that the trajectories intersect with the Galactic disk, and the Galactic centerlies outside of the intersection region at the 3a confidence level. We investigate a number of mechanisms that could be responsible for the ejection of the starand find that it is probably ejected from the Galactic disk by supernova explosion or multiple-body interactions in dense young stellar clusters.