Hot subdwarf Atmospheric parameters, Kinematics, and Origins -- Based on 1587 hot subdwarf stars observed in Gaia DR2 and LAMOST DR7

Abstract

Based on the Gaia DR2 catalogue of hot subdwarf star candidates, we identified 1587 hot subdwarf stars with spectra in LAMOST DR7. We present atmospheric parameters for these stars by fitting the LAMOST spectra with {\sc Tlusty/Synspec} non-LTE synthetic spectra. Combining LAMOST radial velocities and Gaia Early Data Release 3 (EDR3) parallaxes and proper motions, we also present the Galactic space positions, velocity vectors, orbital parameters and the Galactic population memberships of the stars. With our He classification scheme, we identify four groups of He rich hot subdwarf stars in the $T_{\mathrm{eff}}-\log g$ and $T_{\mathrm{eff}}-\log (n\mathrm{He}/n\mathrm{H})$ diagrams. We find two extreme He-rich groups ($e$He-1 and $e$He-2) for stars with $\log (n\mathrm{He}/n\mathrm{H})\ge 0$ and two intermediate He-rich groups ($i$He-1 and $i$He-2) for stars with $-1\le \log (n\mathrm{He}/n\mathrm{H})<0$. We also find that over half of the stars in Group $e$He-1 are thick disk stars, while over half of the stars in Group $e$He-2 correspond to thin disk stars. The disk population fractions of Group $i$He-1 are between those of Group $e$He-1 and $e$He-2. Almost all stars in Group $i$He-2 belong to the thin disk. These differences indicate that the four groups probably have very different origins. Comparisons between hot subdwarf stars in the halo and in the Galactic globular cluster $\omega$ Cen show that only He-deficient stars with $-2.2\le \log (n\mathrm{He}/n\mathrm{H})<-1$ have similar fractions. Hot subdwarfs with $\log (n\mathrm{He}/n\mathrm{H})\ge 0$ in $\omega$ Cen have no counterparts in the thick disk and halo populations, but they appear in the thin disk.

arXiv comment34 pages, 17 figures, Accepted for publication in APJS

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