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

¹ Goethe University Frankfurt, Max-von-Laue-Strasse 1, Frankfurt am Main 60438, Germany
e-mail: vescovi@iap.uni-frankfurt.de
² INFN, Section of Perugia, Via A. Pascoli snc, 06123 Perugia, Italy
³ INAF, Observatory of Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
⁴ Department of Physics and Geology, University of Perugia, Via A. Pascoli snc, 06123 Perugia, Italy
⁵ University of Granada, Departamento de Fisica Teorica y del Cosmos, 18071 Granada, Spain

Received: 23 April 2021
Accepted: 15 June 2021

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.