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 ^*,**

¹ Instituut voor Sterrenkunde, Departement Natuurkunde en Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
² Dipartimento di Fisica Generale, Universitá di Torino, via Pietro Giuria 1, 10125 Torino, Italy
³ Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, 3800 Australia
⁴ Osservatorio Astronomico di Collurania, 64100 Teramo, Italy

Corresponding author: M. Reyniers, Maarten.Reyniers@ster.kuleuven.ac.be

Received: 25 September 2003
Accepted: 18 December 2003

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 06530–0213 and IRAS 08143–4406 were analysed. The objects are moderately metal deficient by 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 [CITE]. 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.