New Hot Subdwarf Stars Identified in Gaia DR2 with LAMOST DR5 Spectra. II.

Abstract

Three hundred and eighty eight hot subdwarf stars have been identified by using the Hertzsprung–Russell (HR) diagram built from the second data release of the Gaia mission. By analyzing their observed Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectra, we characterized 186 sdB, 73 He-sdOB, 65 sdOB, 45 sdO, 12 He-sdO, and 7 He-sdB stars. The atmospheric parameters of these stars (e.g., T eff , log g , log( n He/ n H)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra calculated from non-local thermodynamic equilibrium model atmospheres. Among these stars, we have 135 new identified hot subdwarfs which have not been cataloged before. Although 253 stars appear in the catalog by Geier et al., only 91 of them have atmospheric parameters. Together with the 294 hot subdwarf stars found by Lei et al., we identified 682 hot subdwarf stars in total by using the Gaia HR-diagram and LAMOST spectra. These results demonstrate the efficiency of our method to combine large surveys to search for hot subdwarf stars. We found a distinct gap in our He-sdOB stars based on their He abundance, which is also presented in extreme horizontal branch (EHB) stars of the globular cluster ω Cen. The number fraction of the sample size for the two subgroups is very different between the two counterparts. However, the distinct gap between the H-sdB stars and He-sdOB stars in ω Cen is not visible in our sample. More interestingly, the He-sdB population with the highest He abundance in our sample is completely missing in ω Cen. The discrepancy between our field hot subdwarf stars and the EHB stars in ω Cen indicate different origins for the two counterparts.