A new model for s-process nucleosynthesis in low-mass, low-metallicity AGB stars

We have investigated evolution of low-mass, low-metallicity AGB stars. We evolved models with masses ranging from 1 to 3M_⊙ and metalllicity of [Fe/H]= -2.7. We found for M ≲ 2.5M_⊙ models that in the early phase of the AGB evolution convective shell triggered by thermal runaway of He shell burning extends upwards and reaches into the bottom of the overlying H-rich envelope. Protons are mixed into the hotter He intershell. Proton capture reactions occur under He burning conditions and lead to release of huge amounts of energy. This phenomenon causes the surface chemical composition to change dramatically. We present surface abundance changes of CNO elements and lithium in this event.