1999AJ....117.1834R

A detailed chemical composition analysis of the bright post-AGB candidate HD 179821 (IRAS 19114+0002) is presented. The LTE analysis, based on high-resolution (R~50,000) and high-quality (S/N~300) spectra, yields atmospheric parameters T_eff=6750 K, log g=0.5, and ξ_t=5.25 km.s^–1.

The elemental abundance results of HD 179821 are found to be [Fe/H]=-0.1, [C/Fe]=+0.2, [N/Fe]=+1.3, [O/Fe]=+0.2, [α-process/Fe]=+0.5, and [s-process/Fe]=+0.4. These values clearly differ from the elemental abundances of Population I F supergiants. The C, N, and O abundances and the total CNO abundance value relative to Fe, [C+N+O/Fe]=+0.5, indicate that the photosphere of HD 179821 is contaminated with both the H- and He-burning products of the AGB phase. The evidence for He burning through the 3α process and deep AGB mixing also comes from the observed overabundances of s-process elements. Remarkably, the abundance of the α-process element Na is found to be very large, [Na/Fe]=+0.9.

The ratio O/C=2.6 indicates that the atmosphere is oxygen rich. The results of this abundance study support the argument that HD 179821 is a proto-planetary nebula, probably with an intermediate-mass progenitor. However, the strength of the O I triplet lines at 7774 Å and the distance derived from the interstellar Na I D1 and D2 components imply that the star is a luminous object (M_bol~-8.9±1) and thus a massive supergiant. Thus, while this study contributes important new observational results for this star, an unambiguous determination of its evolutionary status has yet to be achieved.