Chemical composition of optically bright post-AGB stars

We present a detailed LTE chemical analysis of 10 optically bright F-type post-AGB objects on the basis of the analysis of high-resolution optical spectra and compare the results with similar objects discussed in the literature. The iron content is low on average, and so confirms the old and hence low-mass nature of the supergiants, with a noticable exception of HD 95767. We emphasize the fact that the chemical patterns observed are very diverse : several different classes can be distinguished. Only aminor fraction of the objects are conform to standard post third dregde-up theory. Only in HD 187885 (Van Winckel et al., 1996), HD 56126 (Klochkova, 1995) and HD 158616 (this paper) is there conclusive chemical evidence that they occur in a post-AGB evolutionary phase : a high total CNO abundance, for HD 187885 a supersolar He content and–above all–a large overabundance of s-process elements. The other objects, together with other well studied high galactic latitude F-supergiants, display no s-process enhancement but even depletion in some cases. The high N abondance and the mildly enhanced total CNO abundance indicate that the atmospheres of these objects contain a mixture of CNO-cycled material and He-burning products. For some sources, however, this enhancement of the total CNO abundance is barely significant. HD 107369, the only object in our sample with neither Hα emission nor observed IR excess, displays also unique chemical patterns among our sample stars ( a C deficiency coupled with a moderate Fe depletion of [Fe/H] = − 1.1). This star is the only object in our sample showing similar chemical patterns to the metal poor B stars at high galactic latitude (Conlon et al., 1993). Our chemical analysis does therefore not point to an evolutionary connection between the dusty high-latitude supergiants and the metal-poor B stars, but rather suggests that the latter evolve from stars such as HD 107369.