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Issue A&A
Volume 471, Number 1, August III 2007
Page(s) 247 - 254
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20077607



A&A 471, 247-254 (2007)
DOI: 10.1051/0004-6361:20077607

IRAS 08281-4850 and IRAS 14325-6428: two A-type post-AGB stars with s-process enrichment

M. Reyniers1, G. C. Van de Steene2, P. A. M. van Hoof2, and H. Van Winckel1

1  Instituut voor Sterrenkunde, Departement Natuurkunde en Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
2  Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium

(Received 4 April 2007 / Accepted 6 June 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.


Key words: stars: AGB and post-AGB -- stars: abundances -- stars: carbon -- stars: individual: IRAS 14325-6428 -- stars: individual: IRAS 08281-4850



© ESO 2007


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