The Chemical composition of the post-AGB F-supergiant CRL 2688
  We present an analysis of a high resolution (R~50,000) optical spectrum of
the central region of the proto-planetary nebula CRL 2688. This object is
thought to have recently moved off the AGB, and display abundance patterns of
CNO and heavy elements that can provide us with important clues to understand
the nucleosynthesis, dredge-up and mixing experienced by the envelope of the
central star during its AGB stage of evolution. The analysis of the molecular
features, presumably originated from the 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 K, log g=0.5 and [Fe/H]=-0.3 dex. We find that the abundance pattern
of this object is characterized by enhancements of Carbon ([C/Fe]=0.6),
Nitrogen ([N/Fe]=1.0) and Na ([Na/Fe]=0.7), 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, suggesting
that there is no depletion of refractory elements. The Halpha, Na I and K I
resonance lines show prominent emission components, whose helio-centric radial
velocities are offsetted by -41 km/s relative to the photospheric
metal-absorption lines. The molecular features of C_2 and CN also show emission
components, whose velocities are consistent with the emission components of the
Halpha, Na I, and K I lines. On the other hand, their absorption components are
more highly blue shifted than the corresponding emission components, which
suggests that the regions where the emission and absorption components arise
are expanding at different velocities.
