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

Abstract

In this paper, we report the discovery of a new late-B-type unbound hyper-runaway star (LAMOST-HVS4) from the LAMOST spectroscopic surveys. According to its atmospheric parameters, it is either a B-type main-sequence (MS) star or 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 center lies outside of the intersection region at the 3 σ confidence level. We investigate a number of mechanisms that could be responsible for the ejection of the star and find that it is probably ejected from the Galactic disk by supernova explosion or multiple-body interactions in dense young stellar clusters.