Reyniers, M. ; van de Steene, G. C. ; van Hoof, P. A. M. ; van Winckel, H.

published in Astronomy and Astrophysics, 471, pp. 247-254 (2007)

Abstract: Aims:One of the puzzling findings in the study of the chemical evolution of (post-)AGB stars is why very similar stars (in terms of metallicity, spectral type, infrared properties, etc.) show a very different photospheric composition. We aim at extending the still limited sample of s-process enriched post-AGB stars, in order to obtain a statistically large enough sample that allows us to formulate conclusions concerning the 3rd dredge-up occurrence. Methods: We selected two post-AGB stars on the basis of IR colours indicative of a past history of heavy mass loss: IRAS 08281-4850 and IRAS 14325-6428. They are cool sources in the locus of the Planetary Nebulae (PNe) in the IRAS colour-colour diagram. Abundances of both objects were derived for the first time on the basis of high-quality UVES and EMMI spectra, using a critically compiled line list with accurate log(gf) values, together with the latest Kurucz model atmospheres. Results: Both objects have very similar spectroscopically defined effective temperatures of 7750 - 8000 K. They are strongly carbon and s-process enriched, with a C/O ratio of 1.9 and 1.6, and an [ls/Fe] of +1.7 and +1.2, for IRAS 08281-4850 and IRAS 14325-6428 respectively. Moreover, the spectral energy distributions (SEDs) point to heavy mass-loss during the preceding AGB phase. Conclusions: IRAS 08281-4850 and IRAS 14325-6428 are prototypical post-AGB objects in the sense that they show strong post 3rd dredge-up chemical enrichments. The neutron irradiation has been extremely efficient, despite the only mild sub-solar metallicity. This is not conform with the recent chemical models. The existence of very similar post-AGB stars without any enrichment emphasizes our poor knowledge of the details of the AGB nucleosynthesis and dredge-up phenomena. We call for a very systematic chemical study of all cool sources in the PN region of the IRAS colour-colour diagram. Based on observations collected at the European Southern Observatory, Chile (programmes 70.D-0278(A) and 73.D-0241(A)) Tables 4 et 5 are only available in electronic form at http://www.aanda.org

DOI: 10.1051/0004-6361:20077607
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