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

Abstract

388 hot subdwarf stars have been identified by using the Hertzsprung-Russell (HR) diagram built from the second data release (DR2) of the Gaia mission. By analyzing their observed 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(nHe/nH)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra calculated from non-Local Thermodynamic Equilibrium (non-LTE) 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. (2017) , but only 91 of them have atmospheric parameters. Together with the 294 hot subdwarf stars found by Lei et al. (2018), 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 (GC) ω Cen. The number fraction of the sample size for the two sub-groups 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 stas in ω Cen indicate different origins for the two counterparts.

Hot Subdwarf Stars Identified in Gaia DR2 with Spectra of LAMOST DR6 and DR7. I. Single-lined Spectra

“…Two distinct helium sequences, e.g., a He-rich sequence (fitted by dotted line) and a He-weak sequence (fitted by dot-dashed line), which were discovered by Edelmann et al (2003) and confirmed by later studies (Németh et al 2012;Geier et al 2013;Luo et al 2016b;Lei et al 2018Lei et al , 2019b, are clearly present in this panel. As found by Lei et al (2019b), the He-rich sequence consists of sdB, sdOB, He-sdOB and He-sdB stars, while the He-weak sequence consists purely of sdO stars. Furthermore, the 3 He-sdO stars (aqua left triangles) identified in this study are located between the two He sequences.…”

“…In panel b, one also can find a gap (e.g., T eff = 40000 K and log(nHe/nH) = 0.0) in He-sdOB stars (red diamonds), which splits the He-sdOB stars into two subgroups, e.g., a subgroup with higher He abundances and temperatures, and the other subgroup with lower He abundance and temperatures. With larger size of He-sdOB stars, this gap is more clearly present in panel b of Fig 4 in Lei et al (2019b). As discussed in Lei et al (2019b), the gap also appears in the EHB stars of GC ω Cen, but the star fraction of the two subgroups between field hot subdwarf stars and ω Cen EHB stars are very different.…”

“…In our previous work, we have already identified 682 hot subdwarf stars by combining the Gaia DR2 database and the LAMOST DR5 database (Lei et al 2018(Lei et al , 2019b, among which 241 stars were newly discovered. In this study, we analyzed hot subdwarf candidates selected from the LAMOST DR6 and DR7 v0 database.…”

“…As did in Lei et al (2018Lei et al ( , 2019b, we employed the spectral analysis tool, XTgrid (Németh et al 2012(Németh et al , 2014 to analyze the selected 607 spectra. XTgrid fits the observed data with synthetic spectra (Synspec version 49; Lanz et al 2007) calculated from non-LTE model atmospheres (Tlusty version 204; Hubeny & Lanz 2017).…”

“…Parameter uncertainties have been estimated by mapping the ∆χ 2 field until the 60 percent confidence level at the given number of free parameters was reached. For the detailed information to obtain the parameter error bars, the readers are suggested to see Fig 2 in Lei et al (2019b) and the text therein.…”