Spectroscopic study of HF179821 (IRAS 19114+0002): Proto-planetary nebula or supergiant?

A detailed chemical composition analysis of the bright post-AGB candidate H D 179821 (IRAS 19114+0002) is presented. The LTE analysis, based on high-re solution (R approximate to 50,000) and high-quality (S/N approximate to 300 ) spectra, yields atmospheric parameters T-eff = 6750 K, log g = 0.5, and x i(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, [alpha-process/Fe] = +0.5, an d [s-process/Fe] = +0.4. These values clearly differ from the elemental abu ndances of Population I F supergiants. The C, N, and O abundances and the t otal CNO abundance value relative to Fe, [C+N+O/Fe] = +0.5, indicate that t he 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 3 alpha process and deep AGB mixing also comes from the observed overabundances of s-process elements. Remarkably, the abundance of the alpha-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 resul ts of this abundance study support the argument that HD 179821 is a proto-p lanetary nebula,probably with an intermediate-mass progenitor. However, the strength of the O I triplet lines at 7774 Angstrom and the distance derive d from the interstellar Na I D1 and D2 components imply that the star is a luminous object (M-bol similar to -8.9 +/- 1) and thus a massive supergiant . Thus, while this study contributes important new observational results fo r this star, an unambiguous determination of its evolutionary status has ye t to be achieved.