High-resolution Keck I spectroscopy of Galactic halo post-asymptotic giant branch stars

• Published in: Monthly notices of the Royal Astronomical Society Vol. 337; no. 3; pp. 851 - 860
• Main Authors: Mooney, C. J., Rolleston, W. R. J., Keenan, F. P., Dufton, P. L., Smoker, J. V., Ryans, R. S. I., Aller, L. H.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 11-12-2002
• Subjects: stars: abundances; stars: AGB and post-AGB; stars: early-type; stars: Population II
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.2002.05912.x

Absolute and differential abundance analyses have been performed from high-resolution, high signal-to-noise ratio optical (Keck I) spectra for three evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 and LS IV −04 01. Their derived atmospheric parameters indicate that all three objects are undergoing a post-asymptotic giant branch (post-AGB) phase of evolution. A differential abundance analysis reveals HD 341617 as having a mild carbon deficiency of 0.74 dex, possibly due to the star having evolved off the AGB before the onset of the third dredge-up. Although such carbon underabundances are typical of hot post-AGB objects, the same trend is not observed in PG 1704 + 222, where the carbon abundance is found to be consistent with those derived for nitrogen and oxygen. Hence, a dredge-up scenario need not be invoked to explain the chemical composition of PG 1704 + 222. For LS IV −04 01 no iron deficiency is apparent relative to magnesium and silicon, and hence a gas—dust separation event in the AGB progenitor need not be invoked for this star.