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

Rao, S. Sumangala
Pandey, Ganjendra
Lambert, David L.
Giridhar, Sunetra
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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 similar or equal to -0.2 (Fe/H 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.

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.