Hot Subdwarf Stars Observed in Gaia DR2 and LAMOST DR5

Combing Gaia DR2 with LAMOST DR5, we spectroscopically identified 924 hot subdwarf stars, among which 32 stars exhibit strong double-lined composite spectra. We measured the effective temperature T_eff, surface gravity $\mathrm{log}\,g$, 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 ${T}_{\mathrm{eff}}\mbox{--}\mathrm{log}\,g$ diagram with our helium abundance classification scheme and two nearly parallel sequences in the ${T}_{\mathrm{eff}}\mbox{--}\mathrm{log}(y)$ diagram. 3D Galactic space motions and orbits of 747 hot subdwarf stars with (G_BP − G_RP)₀ < −0.36 mag 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 $\mathrm{log}(y)\geqslant 0$ are from the double helium white dwarfs (HeWDs) merger, He-deficient hot subdwarf stars with $-2.2\leqslant \mathrm{log}(y)\lt -1$ from the common envelope ejection, and He-deficient hot subdwarf stars with $\mathrm{log}(y)\lt -2.2$ from the stable Roche lobe overflow channels. The relative number of He-rich hot subdwarf stars with $-1\leqslant \mathrm{log}(y)\lt 0$ and $\mathrm{log}(y)\geqslant 0$ in the halo is more than twice the prediction of Zhang et al. by more than six times in the thin disk, which implies that the mergers of HeWDs with low-mass main-sequence stars may not be the main formation channel for He-rich hot subdwarf stars with $-1\leqslant \mathrm{log}(y)\lt 0$, especially in younger environments.