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Register a new userHot Subdwarf Stars Identified in Gaia DR2 with Spectra of LAMOST DR6 and DR7. II. Kinematics
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Combining the LAMOST radial velocities with Gaia parallaxes and proper motions, we presented 3D Galactic space motions and the orbits of 182 single-lined hot subdwarf stars. These stars have been identified by Lei et al. (2020) in Gaia DR2 with LAMOST DR6 and DR7 spectra. He-rich hot subdwarf stars with log(y)>0 show the largest standard deviations of the Galactic velocity components and orbital parameters, while those with -1<log(y)<0 exhibit the second largest standard deviations. The two groups of He-deficient stars with log(y)<-1 show similar standard deviations, which is systematically lower compared to He-rich stars. We also presented a kinematic population classification of the four hot subdwarf helium groups based on their positions in the U-V velocity diagram, J_z-eccentricity diagram and their Galactic orbits. The overall tendency of the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk and thick disk is largely consistent with the findings reported by Luo et al.(2019) based on LAMOST DR5, which appears to support the predictions of binary population synthesis (Han et al. 2003; 2008). He-deficient stars with -2.2<log(y)<-1 likely origin from stable the Roche lobe overflow channel, He-deficient stars with log(y)<-2.2 from the common envelope ejection channel, and He-rich stars with log(y)>0 from the merger channel of double He white dwarf stars. The fraction of He-rich hot subdwarf stars with -1<log(y)<0 in the thin disk and the halo are far higher than in the thick disk, which implies that these stars have different formation channels in the thin disk and in the halo.
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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 non-local 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 subdwarf 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.
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 $T_{rm eff}-log,g$ and $T_{rm eff}-log{(n{rm He}/n{rm H})}$ diagrams. We find two extreme He-rich groups ($e$He-1 and $e$He-2) for stars with $log{(n{rm He}/n{rm H})}geq0$ and two intermediate He-rich groups ($i$He-1 and $i$He-2) for stars with $-1lelog{(n{rm He}/n{rm H})}<0$. 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 $-2.2lelog{(n{rm He}/n{rm H})}<-1$ have similar fractions. Hot subdwarfs with $log{(n{rm He}/n{rm H})}ge 0$ in $omega$ Cen have no counterparts in the thick disk and halo populations, but they appear in the thin disk.
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 $-2.2lelog(y)<-1$ 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 $-1lelog(y)<0$ 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 $-1lelog(y)<0$, specially in younger environments.
We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 non-composite spectra subdwarf stars were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. Most of the sdB stars scatter near the Extreme Horizontal Branch in the $T_{rm eff}-log{g}$ diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near $T_{rm eff}=45,000$ K and $log(g) = 5.8$. The sdB population separates into several nearly parallel sequences in the $T_{rm eff}-{rm He}$ abundance diagram with clumps corresponding to those in the $T_{rm eff}-log{g}$ diagram. Over $38,000$ K (sdO) stars show abundance extremes, they are either He-rich or He-deficient and we observe only a few stars in the $ -1 < log(y) < 0$ abundance range. With increasing temperature these extremes become less prominent and the He abundance approaches to $log(y)sim-0.5$. A unique property of our sample is that it covers a large range in apparent magnitudes and galactic latitudes, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch (2009) and we conclude that He-rich and He-deficient sdB stars ($log(y) < 1$) probably origin from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the luminosity and helium abundance plane, which suggests that these atmospheric parameters are correlated.
Employing a new machine learning method, named hierarchical extreme learning machine (HELM) algorithm, we identified 56 hot subdwarf stars in the first data release (DR1) of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) survey. The atmospheric parameters of the stars are obtained by fitting the profiles of hydrogen (H) Balmer lines and helium (He) lines with synthetic spectra calculated from non-Local Thermodynamic Equilibrium (NLTE) model atmospheres. Five He-rich hot subdwarf stars were found in our sample with their log(nHe/nH) > -1 , while 51 stars are He-poor sdB, sdO and sdOB stars. We also confirmed the two He sequences of hot subdwarf stars found by Edelmann et al. (2003) in Teff - log(nHe/nH) diagram. The HELM algorithm works directly on the observed spectroscopy and is able to filter out spectral properties without supplementary photometric data. The results presented in this study demonstrate that the HELM algorithm is a reliable method to search for hot subdwarf stars after a suitable training is performed, and it is also suitable to search for other objects which have obvious features in their spectra or images.
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