Abstract
Objects that may be in a post-AGB evolutionary stage include high-latitude supergiants, UU Her and RV Tau variables, and IRAS sources identified with A- to G-type supergiants. Photospheric abundance analyses of such objects typically reveal moderate to extreme iron-group deficiencies, consistent with membership in the thick-disk or halo populations, and with having arisen from low-mass progenitors. The photospheric CNO abundance patterns in such stars are distinctly atypical of normal Population I supergiants, and appear to indicate the presence of helium-burning products at the stellar surface. However, enhancements of s-process elements are typically not seen, suggesting that most of the stars have avoided the AGB dredge-up. A subset of the stars, typified by HR 4049, show ultra-low iron-group abundances and extreme enhancements of C, N, O, and S. They appear to constitute a new class of chemically peculiar stars, with severely depleted photospheric metals. Grain formation is proposed as the depletion mechanism.
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