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Analysis of helium-rich subdwarf O stars. 1: NLTE models, methods, and FITS for 21 Palomar Green survey sdOs
Thejll, P.; Bauer, F.; Saffer, R.; Liebert, J.; Kunze, D.; Shipman, H. L.
AA(Niels Bohr Institute, Copenhagen, Denmark), AB(Institut für Astronomie und Astrophysik der Universität München, München, Germany), AC(Space Telescope Science Institute, Baltimore, MD), AD(Steward Observatory, University of Arizona, Tucson, AZ), AE(Institut für Astronomie und Astrophysik der Universität München, München, Germany), AF(University of Delaware, Newark, DE)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 433, no. 2, p. 819-830 (ApJ Homepage)
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NASA/STI Keywords:
Abundance, Helium, Local Thermodynamic Equilibrium, O Stars, Sky Surveys (Astronomy), Stellar Atmospheres, Stellar Models, Stellar Spectra, Subdwarf Stars, Distance, Galactic Halos, Kinematics, Stellar Luminosity, Stellar Mass, Stellar Temperature
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Atmospheric parameters for 21 helium-rich hot subdwarf O stars from the Palomar Green survey are found from fits of non-local thermodynamic equilibrium (NLTE) models to optical spectra. About 250 new NLTE models in the parameter range Teff from 35,000 to 65,000 K, log (g) from 4.0 to 6.5, and epsilon(He) from 50% He to 99% He have been calculated. A fit for each object is presented. Estimated distances and luminosities are calculated, assuming a mass of 0.5 solar masses. Large distances above the Galactic plane are found, and this implies that the majority of the sdO stars belong to a different stellar population than the sdB, planetary nebulae and white dwarf stars. Possibilities may include the halo and a 'thick disk.' Kinematical data from the literature are also discussed in view of the found distances. Five of the stars have been observed, modeled, and analyzed by Dreizler et al., and significant differences exist between their results and ours for these stars. The reason for these differences is not yet known, but the problem does not alter the conclusions about population membership that we present.

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