Nucleosynthesis of s-elements in zero-metal AGB stars

Contrary to previous expectations, recent evolutionary models of zero-metal licity stars show that the development of mixing episodes at the beginning of the AGB phase allows low- and intermediate-mass stars to experience ther mal pulses. If these stars, like their metal-rich counterparts, also experi ence partial mixing of protons from the H-rich envelope into the C-rich lay ers at the time of the third dredge-up, an extensive neutron capture nucleo synthesis leads to the production of s-process nuclei up to Pb and Bi. Nucl eosynthesis calculations based on stellar AGB models are performed assuming a parameterized H-abundance profile below the convective envelope at the t ime of the third dredge-up. Despite the absence of Fe-group elements, the l arge neutron flux resulting from the C-13(alpha ,n) O-16 reaction leads to an efficient production of s-process elements starting from the neutron cap tures on the C-Ne isotopes. Provided partial mixing of protons takes place, it is shown that population III AGB stars should be enriched in s-process elements and overall in Pb and Bi.