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The chemical composition of the protoplanetary nebula candidate HD 179821
A high-resolution and signal-to-noise ratio spectrum has been obtainedfor the protoplanetary nebula candidate HD 179821=IRAS 19114+0002 usingthe Academy of Sciences Astrophysical Observatory 6-m telescope and CCDdetector. The spectroscopic indicators provide the atmosphericparameters T_eff=6800K and logg=1.3 (cgs), approximately equal to thosefor normal F5 supergiants. A standard local thermodynamic equilibrium(LTE) analysis by the model atmospheres method using these parametersshows that most of the iron-peak elements are at most only slightlydeficient ([X]~=-0.10 dex) relative to the normal red giant epsilonVir,however, Sc and Ti are overabundant (~0.5 dex). The alpha-processelements are solar or slightly deficient, except Na, which isoverabundant. Carbon and elements heavier than Ni are enhanced by +0.7dex on average compared with the standard star. The heliocentric radialvelocity is found to be +88.4 km s^-1. The detached circumstellar shell,its radial velocity, and the likely overabundance of carbon ands-process elements suggest that HD 179821 is probably an evolvedlow-mass star in the evolutionary stage between the asymptotic giantbranch (AGB) and planetary nebulae (PNe).
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