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

Deng

et al. 2019

ApJ

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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 log g, helium abundance y = nHe/nH, 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 eff − log g diagram with our helium abundance classification scheme and two nearly parallel sequences in the T 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) ≥ 0 are from the double helium white dwarfs merger, He-deficient hot subdwarf stars with −2.2 ≤ 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 ≤ log(y) < 0 and log(y) ≥ 0 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 ≤ log(y) < 0, specially in younger environments.

Section: Spectral Analysissupporting

confidence: 76%

Hot Subdwarf Stars Observed in Gaia DR2 and LAMOST DR5

Luo

Deng

et al. 2019

ApJ

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“…It means that we not only identified 241 new hot subdwarf stars, but also newly obtained atmospheric parameters for 255 hot subdwarf stars which were already cataloged in Geier et al (2017a). These results indicate the efficiency of our method to select hot subdwarf candidates by cross-matching these two large survey 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 logg(cm s 2 ) 5 4 3 2 1 0 1 2 3 4 log(nHe/nH) Hot subdwarfs in this study and Lei et al (2018) He-sdB n=9 He-sdO n=19 He-sdOB n=104 sdB n=355 sdO n=67 sdOB n=128 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 logg(cm s 2 )…”

Section: Discussionsupporting

confidence: 65%

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

Lei

Zhao

et al. 2019

ApJ

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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.

“…Therefore, we cross-matched all the 864 hot subdwarf sars identified in our series of studies with the Geier et al (2019) catalog, and found 833 common stars. This result demonstrate that nearly all the hot subdwarf stars identified in Lei et al (2018Lei et al ( , 2019b are included in Geier et al (2019) catalog. As described in section 2.3, 2513 candidates from the catalog of Geier et al (2019) have LAMOST spectra, of which 1348 have SNR-u larger than 10, and 833 of them were spectroscopically identified as hot subdwarf stars.…”

Section: Discussion and Summarymentioning

confidence: 55%

“…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.…”

Section: Catalogue With Lamost Dr6 and Dr7mentioning

confidence: 99%

“…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).…”

Section: Spectroscopy and Spectral Classificationmentioning

confidence: 99%

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

Lei

Zhao²,

et al. 2020

ApJ

Self Cite

182 single-lined hot subdwarf stars are identified by using spectra from the sixth and seventh data release (DR6 and DR7) of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) survey. We classified all the hot subdwarf stars using a canonical classification scheme, and got 89 sdB, 37 sdOB, 26 sdO, 24 He-sdOB, 3 He-sdO and 3 He-sdB stars, respectively. Among these stars, 108 hot subdwarfs are newly discovered, while 74 stars were reported by previous catalogs. The atmospheric parameters of these stars were obtained by fitting the hydrogen (H) and helium (He) lines with nonlocal thermodynamic equilibrium (non-LTE) model atmospheres. The atmospheric parameters confirm the two He sequences and the two subgroups of He-sdOB stars in our samples, which were found by previous studies in the T eff -log(nHe/nH) diagram. Our results demonstrate different origins of field hot subdwarf stars and extreme horizontal branch (EHB) stars in globular clusters (GCs), and provide strict observational limits on the formation and evolution models of the different sub-types of these evolved objects. Based on the results, we evaluated the completeness of the Geier et al. (2019) catalog. We found the fraction of hot subwarf stars is between 10% and 60%, depending on the brightness of the sample. A more accurate estimation for the hot subdwarf fraction can be obtained when similar results from composite spectra will become available.