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 at close-to-solar metallicity observations and theory agree, at lower metallicities stellar models overestimate the fluorine production with respect to heavy elements. We present 19 F 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 13 C neutron source (the so-called 13 C pocket). We find that 19 F is mainly produced as a result of nucleosynthesis involving secondary 14 N during convective thermal pulses, with a negligible contribution from the 14 N present in the 13 C pocket region. A large 19 F production is thus prevented, resulting in lower fluorine surface abundances. As a consequence, AGB stellar models with magnetic-buoyancy-induced mixing at the base of the convective envelope well agree with available fluorine spectroscopic measurements at both low and close-to-solar metallicity.

Planetary Nebulae: Sources of Enlightenment

“…They conclude from their studies, however, that AGB stars are not the main contributors of F in the solar neighborhood (core collapse SNe are) in agreement with chemical evolution models extant in the literature, e.g., Kobayashi et al (2011). Further analysis by this same team presented in Vescovi et al (2021) also finds that F abundance in AGB stars is directly related to the average s-process abundance <s> such that [F/Fe] and [<s>/Fe] increase together (see their Figure 2, upper panel). Overall, they also find that their stellar models better reproduce the F/H abundance observations when magnetic buoyancy "acts as the driving force for the formation of the 13 C neutron source" in metal-poor AGB stars during TDU.…”

The most metal-rich stars in the universe: chemical contributions of low and intermediate mass asymptotic giant branch stars with metallicities between $0.04 \leq Z \leq 0.10.$

“…We note that the work of Abia et al (2019) and Vescovi et al (2021) have shown recently that the contributions of 19 F from AGB stars are a significantly smaller percentage of the budget than originally predicted by Jorissen et al (1992). Vescovi et al (2020Vescovi et al ( , 2021 incorporate magnetically induced 13 C pockets in their models which result in lower proton abundances. They thus produce less 14 N and 19 F than models with 13 C pockets produced by convective overshoot.…”