Buoyant magnetic flux tubes as a site for 26Al production in AGB stars

We address the diffusive circulation of matter occurring in AGB stars above the H-burning shell, which triggers the production of p-rich isotopes like 13C and 17O and of the unstable nucleus 26Al, observed in presolar grains and in Early Solar System materials. As a physical mechanism for these phenomena we consider the buoyancy of magnetic flux tubes formed below the convective envelope and we arrive at expressions for the relevant mixing parameters in terms of the required strength of the magnetic field |/rightarrowB|. We show how values of |/rightarrowB| like those normally expected in the radiative layers above the H-burning shell of a red giant can indeed trigger chemical mixing at the required efficiency and should therefore be considered for explaining the abundance peculiarities induced in such stars by proton captures. The technique discussed here is suitable for chemically stratified regions. A different formalism should be used for studying the more internal, well-mixed He layers, where similar circulation phenomena induce the formation of the neutron source 13Cb during dredge-up. ...