A homogeneous study of the s-process in the 21 mu m carbon-rich post-AGB objects

We present in this paper a homogeneous photospheric abundance study, on the basis of the analysis of high resolution optical spectra, of six post-AGE objects displaying a 21 mu m circumstellar dust feature in their IR spectru m. The F-G spectral type of the 21 mu m stars make that a large range of el ements including a wide variety of s-process elements, can be studied by th eir atomic lines. The high C/O-ratios together with the large overabundance of s-process elements prove that the objects are descendants of field carb on stars. We discuss in detail the s-process abundance distribution display ed by these 21 mu m stars and conclude that the 3rd dredge-up efficiency is closely related to the strength of the integrated neutron irradiation. The expected anti-correlation of the neutron irradiation with metallicity, on the other hand, contains a large intrinsic scatter. Finally we compare our results with other intrinsic and extrinsic s-process enriched objects and c onclude that the post-AGB stars offer very useful complementary data to con strain the evolutionary models of AGE nucleosynthesis and dredge-up process es.