Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities

Abstract

Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models at lower metallicities overestimate the fluorine production with respect to that of heavy elements. We present 19F nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the 13C neutron source (the so-called 13C pocket). We find that 19F is mainly produced as a result of nucleosynthesis involving secondary 14N during convective thermal pulses, with a negligible contribution from the 14N present in the 13C pocket region. A large 19F production is thus prevented, resulting in lower fluorine surface abundances. As a consequence, AGB stellar models with mixing induced by magnetic buoyancy at the base of the convective envelope agree well with available fluorine spectroscopic measurements at low and close-to-solar metallicity.