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Register a new userM Subdwarf Research. II. Atmospheric Parameters and Kinematics
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Applying the revised M subdwarf classification criteria discussed in Paper I to LAMOST DR7, combining the M subdwarf sample from Savcheva et al, a new M subdwarf sample was constructed for further study. The atmospheric parameters for each object were derived fitting with the PHOENIX grid, combining with Gaia DR2, the relationship between the gravity and metallicity were explored according to the locus both in the color-absolute magnitude diagram and the reduced proper motion diagram. Objects that have both the largest gravity and the lowest metallicity are located away from the main-sequence cloud and may be considered as the intrinsic M subdwarfs, which can be classified as luminosity class VI. Another group of objects whose spectra show typical M subdwarf characters have lower gravity and relatively moderate metal deficiency and occupy part of the ordinary M dwarf region in both diagrams. The Galactic U , V , W space velocity components and their dispersion show that the local Galactic halo population sampled in the solar neighborhood is represented by objects of high gravity and an inconspicuous bimodal metallicity distribution, with a fraction of prograde orbits. The other M subdwarfs seem to partly belong to the thick disk component with a significant fraction of thin disk moderately metal-poor objects intricately mixed with them. However, the selection effects, especially the favored anti-center direction of investigation in the LAMOST sub-sample, but also contamination by multiplicity and parameter coupling could play important roles and need to be further investigated.
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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.
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.
The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding sdBs with compact companions like supermassive white dwarfs (M>1.0 Msun), neutron stars or black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they are likely to exist in our Galaxy. A determination of the orbital parameters is sufficient to put a lower limit on the companion mass by calculating the binary mass function. If this lower limit exceeds the Chandrasekhar mass and no sign of a companion is visible in the spectra, the existence of a massive compact companion is proven without the need for any additional assumptions. We identified about 1100 hot subdwarf stars from the SDSS by colour selection and visual inspection of their spectra. Stars with high velocities have been reobserved and individual SDSS spectra have been analysed. In total 127 radial velocity variable subdwarfs have been discovered. Binaries with high RV shifts and binaries with moderate shifts within short timespans have the highest probability of hosting massive compact companions. Atmospheric parameters of 69 hot subdwarfs in these binary systems have been determined by means of a quantitative spectral analysis. The atmospheric parameter distribution of the selected sample does not differ from previously studied samples of hot subdwarfs. The systems are considered the best candidates to search for massive compact companions by follow-up time resolved spectroscopy.
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.
The asteroseismic and planetary studies, like all research related to stars, need precise and accurate stellar atmospheric parameters as input. We aim at deriving the effective temperature (Teff), the surface gravity (log g), the metallicity ([Fe/H]), the projected rotational velocity (v sin i) and the MK type for 169 F, G, K, and M-type Kepler targets which were observed spectroscopically from the ground with five different instruments. We use two different spectroscopic methods to analyse 189 high-resolution, high-signal-to-noise spectra acquired for the 169 stars. For 67 stars, the spectroscopic atmospheric parameters are derived for the first time. KIC 9693187 and 11179629 are discovered to be double-lined spectroscopic binary systems. The results obtained for those stars for which independent determinations of the atmospheric parameters are available in the literature are used for a comparative analysis. As a result, we show that for solar-type stars the accuracy of present determinations of atmospheric parameters is +/- 150 K in Teff, +/- 0.15 dex in [Fe/H], and +/-? 0.3 dex in log g. Finally, we confirm that the curve-of-growth analysis and the method of spectral synthesis yield systematically different atmospheric parameters when they are applied to stars hotter than 6,000 K.
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