Hot Subdwarf Stars Identified in Gaia DR2 with Spectra of LAMOST DR6 and DR7. II.Kinematics

Abstract

Abstract Combining LAMOST radial velocities with Gaia parallaxes and proper motions, we present 3D Galactic space motions and the orbits of 182 single-lined hot subdwarf stars. These stars were identified by Lei et al. in Gaia DR2 with LAMOST DR6 and DR7 spectra. He-rich hot subdwarf stars with show the largest standard deviations of the Galactic velocity components and orbital parameters, while those with exhibit the second largest standard deviations. The two groups of He-deficient stars with show similar standard deviations, which are systematically lower compared to He-rich stars. We also present a kinematic population classification of the four hot subdwarf helium groups based on their positions in the U − V velocity diagram, the eccentricity diagram, and their Galactic orbits. The overall tendency of the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk, and thick disk is largely consistent with the findings reported by Luo et al. based on LAMOST DR5, which appears to support predictions of binary population synthesis. He-deficient stars with likely originate from a stable Roche lobe overflow channel, He-deficient stars with from the common envelope ejection channel, and He-rich stars with from the merger channel of double He white dwarf stars. The fraction of He-rich hot subdwarf stars with in the thin disk and the halo is far higher than that in the thick disk, which implies that these stars have different formation channels in the thin disk and in the halo.