Hot subdwarf stars from LAMOST DR5 & Gaia DR2

We measured the effective temperature T_eff_, surface gravity logg, helium abundance y=nHe/nH, and radial velocities (RVs) of 892 non-composite spectra hot subdwarf stars by fitting LAMOST observations with TLUSTY/SYNSPEC non-LTE synthetic spectra. We outlined four different groups in the T_eff_--logg diagram with our helium abundance classification scheme and two nearly parallel sequences in the T_eff_--log(y) diagram. 3D Galactic space motions and orbits of 747 hot subdwarf stars with (G_BP_-G_RP_)_0_{<}-0.36mag were computed using LAMOST RVs and Gaia parallaxes and proper motions.

Based on the U-V velocity diagram, J_z_-eccentricity diagram, and Galactic orbits, we derived Galactic population classifications and the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk, and thick disk. Comparisons with the predictions of binary population synthesis calculations suggest that He-rich hot subdwarf stars with log(y)>=0 are from the double helium white dwarfs (HeWDs) merger, He-deficient hot subdwarf stars with -2.2<=log(y){<}-1 from the common envelope ejection, and He-deficient hot subdwarf stars with log(y){<}-2.2 from the stable Roche lobe overflow channels.