The High-velocity Stars in the Local Stellar Halo from Gaia and LAMOST

Based on the first Gaia data release and spectroscopy from the LAMOST Data Release 4, we study the kinematics and chemistry of the local halo stars. The halo stars are identified kinematically with a relative speed of at least 220 km s(-1) with respect to the local standard of rest. In total, 436 halo stars are identified. From this halo sample, 16 high-velocity (HiVel) stars are identified. We studied the metallicity and [alpha/Fe] distribution of these HiVel stars. Though most HiVel stars are metal-poor, there are several stars that have metallicities above -0.5 dex. To understand the origin of high-velocity stars, we evolve the trajectory of the star backward along the orbit in our adopted Galaxy potential model to determine the orbital parameters and assess whether the star could have originated in the Galactic center (GC). We found that some high-velocity stars could have originated from the GC, but other stars were probably kicked up from the Galactic disk.