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    Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

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    Author
    Latour, M. cc
    Chayer, P.
    Green, E. M.
    Irrgang, A.
    Fontaine, G.
    Affiliation
    Univ Arizona, Steward Observ
    Issue Date
    2018-01-19
    Keywords
    stars: atmospheres
    stars: abundances
    subdwarfs
    stars: fundamental parameters
    
    Metadata
    Show full item record
    Publisher
    EDP SCIENCES S A
    Citation
    Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars 2018, 609:A89 Astronomy & Astrophysics
    Journal
    Astronomy & Astrophysics
    Rights
    © ESO, 2018.
    Collection Information
    This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.
    Abstract
    Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized. Aims. We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK+81 degrees 266, and LS II + 18 degrees 9, among which the first three are used as standard stars for flux calibration. Our goal is to determine their atmospheric parameters, chemical properties, and evolutionary status to better understand this class of stars that are en route to become white dwarfs. Methods. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. Results. The four bright sdO stars have very similar atmospheric parameters with T-eff between 60 000 and 63 000 K and log g (cm s(-2)) in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S are depleted. The stars have very similar abundances, although AGK + 81 degrees 266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s(-1). The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and HIPPARCOS distances are in good agreement for our three brightest stars. Conclusions. We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.
    Note
    Open access journal.
    ISSN
    0004-6361
    1432-0746
    DOI
    10.1051/0004-6361/201731496
    Version
    Final published version
    Sponsors
    Alexander von Humboldt Foundation; Canada Research Chair; NASA [NAS5-26555]; NASA Office of Space Science [NNX09AF08G]
    Additional Links
    https://www.aanda.org/10.1051/0004-6361/201731496
    ae974a485f413a2113503eed53cd6c53
    10.1051/0004-6361/201731496
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