Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 micrometer star J004441.04-732136.4

By virtue of their spectral types, favourable bolometric corrections as well as their constrained distances, post-AGB stars in external galaxies offer unprecedented tests to AGB nucleosynthesis and dredge-up predictions. We focus here on one object J004441.04-732136.4, which is the only known 21 micrometer source of the SMC. Spectral abundance results reveal J004441.04-732136.4 to be one of the most s-process enriched objects found up to date, while the photospheric C/O ratio of 1.9 +- 0.7, shows the star is only modestly C-rich. J004441.04-732136.4 also displays a low [Fe/H] = -1.34 +- 0.32, which is significantly lower than the mean metallicity of the SMC. From the SED, a luminosity of 7600 +- 200 solar luminosities is found, together with E(B-V) = 0.64 +- 0.02. According to evolutionary post-AGB tracks, the initial mass should be approximately 1.3 solar masses. The photometric variability shows a clear period of 97.6 +- 0.3 days. The detected C/O as well as the high s-process overabundances (e.g. [Y/Fe] = 2.15, [La/Fe] = 2.84) are hard to reconcile with the predictions. The chemical models also predict a high Pb abundance, which is not compatible with the detected spectrum, and a very high 12C/13C, which is not yet constrained by observations. The predictions are only marginally dependent on the evolution codes used. We show that our theoretical predictions match the s-process distribution, but fail in reproducing the detected high overabundances and predict a high Pb abundance which is not detected. Additionally, there remain serious problems in explaining the observed pulsational properties of this source.