Stellar population synthesis of post - AGB stars : the s - process in MACHO 47 . 2496 . 8

Context. The low-metallicity RV Tauri star MACHO 47.2496.8, recently discovered in the Large Magellanic Cloud, is highly enriched in carbon and heavy elements produced by the slow neutron capture process (s-process), and is most probably a genuine post-C(Ntype) asymptotic giant branch (AGB) star. The intrinsic interpretation of the enrichement is further strengthened by detection of a significant infrared excess. The circumstellar dust is the relic of a recent episode of heavy mass loss. We use the analysis of the abundances of MACHO 47.2496.8 to constrain free parameters in AGB models. Aims. We test which values of the free parameters describing uncertain physical mechanisms in AGB stars, namely the third dredgeup and the features of the 13C neutron source, produce models that better match the abundances observed in MACHO 47.2496.8. Methods. We carry out stellar population synthesis coupled with s-process nucleosynthesis using a synthetic stellar evolution code. Results. The s-process ratios observed in MACHO 47.2496.8 can be matched by the same models that explain the s-process ratios of Galactic AGB and post-AGB stars of metallicity > Z⊙/10, except for the choice of the effectiveness of C as a neutron source, which has to be lower by roughly a factor of 3 to 6. The less effective neutron source for lower metallicities is also required when comparing population synthesis results to observations of Galactic halo s-enhanced stars, such as Pb stars. The C/C ratio in MACHO 47.2496.8 cannot be matched simultaneously and requires the occurrence of extra-mixing processes. Conclusions. The confirmed trend of the decreased efficiency of the 13C neutron source with metallicity requires an explanation from AGB s-process models. The present work is to date the first comparison between theoretical models and the detailed abundances of an extragalactic post-AGB star.