Chemical composition of optically bright post-AGB stars *
Hans van Winckel
Received 9 April 1996 / Accepted 19 August 1996
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 a minor 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 H 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.
Key words: stars: abundances stars: evolution stars: individual: SAO 173329, HD 95767, SAO 239853, HD 107369, HD 108015, HD 112374, HD 131356, HD 133656, HD 158616, HD 187885 stars: AGB and post-AGB
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© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998