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The chemical composition of the post-asymptotic giant branch F supergiant CRL 2688




We present an analysis of a high-resolution (R ∼ 50 000) optical spectrum of the central region of the protoplanetary nebula CRL 2688. This object is thought to have recently moved off the asymptotic giant branch (AGB) and displays abundance patterns of CNO and heavy elements that can provide us with important clues to help us understand the nucleosynthesis, dredge-up and mixing experienced by the envelope of the central star during its AGB stage of evolution. Analysis of the molecular features, presumably originating from circumstellar matter, provides further constraints on the chemistry and velocity of the expanding shell, expelled as a consequence of the strong mass loss experienced by the central star.

We confirm that the central star shows a spectrum typical of an F-type supergiant with Teff = 7250 ± 400 K, log g = 0.5 and [Fe/H] = −0.3 ± 0.1 dex. We find that the abundance pattern of this object is characterized by enhancements of carbon ([C/Fe] = 0.6 ± 0.1), nitrogen ([N/Fe] = 1.0 ± 0.3) and Na ([Na/Fe] = 0.7 ± 0.1), similar to other previously known carbon-rich post-AGB stars. Yttrium is also enhanced, while the [Ba/Y] ratio is very low (−1.0), indicating that only the light s-process elements are enhanced. The zinc abundance is found to be normal, [Zn/Fe] = 0.0 ± 0.3, suggesting that there is no depletion of refractory elements. The Hα, Na i and K i resonance lines show prominent emission components, the heliocentric radial velocities of which are offset by −41 ± 3 km s−1 relative to the photospheric metal-absorption lines. The molecular features of C2 and CN also show emission components, the velocities of which are consistent with the emission components of the Hα, Na i and K i lines. On the other hand, their absorption components are more highly blueshifted than the corresponding emission components, which suggests that the regions where the emission and absorption components arise are expanding at different velocities.