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    Is the Post-AGB Star SAO 40039 Mildly Hydrogen-Deficient?

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    2011_08_Rao.pdf (279.5Kb)
    Date
    2011-08
    Author
    Rao, S. Sumangala
    Pandey, Ganjendra
    Lambert, David L.
    Giridhar, Sunetra
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    Abstract
    We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong He I lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of Fe I and Fe II lines, and the wings of the Paschen lines. On the assumption that the He I lines are of photospheric and not chromospheric origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed He I triplet lines at 5876, 4471, and 4713 angstrom, and singlet lines at 4922 and 5015 angstrom. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, I. e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H] = -0.4 dex. But the star's intrinsic metallicity as estimated from Na, S, and Zn, elements of a low condensation temperature, is [Fe/H](o) similar or equal to -0.2 ([Fe/H](o) refers to the star's intrinsic metallicity). The star is enriched in N and perhaps O as well, changes reflecting the star's AGB past and the event that led to He enrichment.
    Department
    McDonald Observatory
    Subject
    stars: abundances
    stars: atmospheres
    stars: evolution
    stars:
    fundamental parameters
    extreme helium stars
    evolutionary status
    cno abundances
    supergiants
    line
    spectroscopy
    spectrometer
    elements
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/42919
    Citation
    Rao, S. Sumangala, Gajendra Pandey, David L. Lambert, and Sunetra Giridhar. "Is the Post-AGB Star SAO 40039 Mildly Hydrogen-Deficient?." The Astrophysical Journal Letters, Vol. 737, No. 1 (Aug., 2011): L7.
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    • facebook
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    © The University of Texas at Austin