IS THE POST-AGB STAR SAO 40039 MILDLY HYDROGEN-DEFICIENT?

Journal Article · Wed Aug 10 04:00:00 EDT 2011 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/737/1/L7· OSTI ID:21562491

Rao, S Sumangala; Pandey, Gajendra; Giridhar, Sunetra ^[1]

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1. Indian Institute of Astrophysics, Bengaluru-560034 (India)

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 A, and singlet lines at 4922 and 5015 A. 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]{sub o} {approx_equal} -0.2 ([Fe/H]{sub 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.

• OSTI ID: 21562491
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 737; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
ATMOSPHERES
ELEMENTS
EQUILIBRIUM
EVOLUTION
FLUIDS
GASES
HELIUM
HYDROGEN
IONIZATION
LTE
NONMETALS
RARE GASES
STAR EVOLUTION
STARS
STELLAR ATMOSPHERES