A study of the s-process in the carbon-rich post-AGB stars IRAS 06530-0213 and IRAS 08143-4406 on the basis of VLT-UVES spectra

Abstract

In an effort to extend the still limited sample of s-process enriched post-AGB stars, high-resolution, high signal-to-noise VLT+UVES spectra of the optical counterparts of the infrared sources IRAS 065 30-0213 and IRAS 08143-4406 were analysed. The objects are moderately metal deficient by [Fe/H] = -0.5 and -0.4 respectively, carbon-rich and, above all, heavily s-process enhanced with a [ls/Fe] of 1.8 and 1.5 respectively. Especially the spectrum of IRAS 06530-0213 is dominated by transitions of s-process species, and therefore resembling the spectrum of IRAS 05341+0852, the most s-process enriched object known so far. The two objects are chemically very similar to the 21 μm objects discussed in Van Winckel & Reyniers (2000). A homogeneous comparison with the results of these objects reveals that the relation between the third dredge-up efficiency and the neutron nucleosynthesis efficiency found for the 21 μm objects, is further strengthened. On the other hand, a detailed comparison with the predictions of the latest AGB models indicates that the observed spread in nucleosynthesis efficiency is certainly intrinsic, and proves that different 13C pockets are needed for stars with comparable mass and metallicity to explain their abundances.