# Hot subdwarf stars observed in Gaia DR2 and LAMOST DR5

## TL;DR

This study combines Gaia DR2 and LAMOST DR5 data to identify and analyze 924 hot subdwarf stars, exploring their physical properties, Galactic distribution, and formation channels, revealing new insights into their origins and population characteristics.

## Contribution

It provides the first large spectroscopic sample of hot subdwarfs with detailed parameter measurements and Galactic population analysis, and compares findings with binary evolution models to infer formation channels.

## Key findings

- Identified 924 hot subdwarfs, including 32 with double-lined spectra.
- Classified four groups in the $T_{eff}-\,log g$ diagram based on helium abundance.
- Discovered that the number ratio of certain helium-rich subdwarfs exceeds model predictions.

## Abstract

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 $T_{\rm eff}$, surface gravity $\log\,g$, helium abundance $y=n{\rm He}/n{\rm H}$, and radial velocities 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 $T_{\rm eff}-\log\,g$ diagram with our helium abundance classification scheme and two nearly parallel sequences in the $T_{\rm eff}-\log(y)$ 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 radial velocities 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 (Han 2008) suggest that He-rich hot subdwarf stars with $\log(y)\ge0$ are from the double helium white dwarfs merger, He-deficient hot subdwarf stars with $-2.2\le\log(y)<-1$ from the common envelope ejection, and He-deficient hot subdwarf stars with $\log(y)<-2.2$ from the stable Roche lobe overflow channels. The relative number of He-rich hot subdwarf stars with $-1\le\log(y)<0$ and $\log(y)\ge0$ in the halo is more than twice the prediction of Zhang et al.(2017), even more than six times in the thin disk, which implies that the mergers of helium white dwarfs with low mass main sequence stars may not be the main formation channel of He-rich hot subdwarf stars with $-1\le\log(y)<0$, specially in younger environments.