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B.E. Reddy (Valparaiso University), Bruce J. Hrivnak (Valparaiso University), E.J. Bakker (University of Texas)
We present an LTE abundance analysis of three post-AGB candidates: IRAS~Z02229+6208, IRAS~07430+1115, and IRAS~19114+0002, based on high-resolution spectra (\lambda/\Delta\lambda\approx 50,000).
Abundance results show that IRAS~Z022229 and IRAS~07430 are slightly metal-deficient ([Fe/H]=-0.5) and overabundant in C, N, and s-process elements. This is in general agreement with the expected nucleosynthesis and deep convective mixing during the preceding AGB phase. Most importantly, both the stars are found to have a high abundance of Li. Thus our results suggest that these two stars may be successors of the small group of carbon-rich and Li-rich AGB stars. We also present an analysis of the circumstellar molecular absorption spectra (C2 & CN) of both stars. The derived expansion velocities, Vexp~=~-11.3 km s-1 and -10.7 km s-1 for IRAS~Z02229 and IRAS~07430, respectively, are typical of values found for post-AGB stars. This brings to eight the number of post-AGB stars for which clear C,N, and s-process elemental over abundances are found.
The elemental abundance results of IRAS~19114 are found to be [Fe/H]=-0.1, [C/Fe]=+0.2, [N/Fe]=+1.1, [\alpha-process/Fe]=+0.5, and s-process elements [Y+Zr+La+Ce+Nd/Fe]=+0.4. The values of total CNO elements ([C+N+O/Fe]=+0.5), s-process elements, and the ratio O/C=2.6 suggest that the photosphere of IRAS~19114 is contaminated with both the H- and He-burning products and HD~179821 may be an intermediate-mass post-AGB star. However, the strength of the O~I triplet lines at 7774Å\ and the interstellar Na~I D1 & D2 components indicate that the star is a more luminous object (Mbol~- 8.9± 1), and therefore a massive supergiant. Thus, while this study of IRAS~19114 contributes important new observational data, an unambiguous determination of its evolutionary status has yet to be determined.
This research was supported by grants to B.J.H from NASA (NAG5-1223) and the NSF (AST-9315107)
The author(s) of this abstract have provided an email address for comments about the abstract: ereddy@kepler.valpo.edu