Letter to the Editor

Magnetic-buoyancy-induced mixing in AGB stars: a theoretical explanation of the non-universal relation of [Y/Mg] to age

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: laura.magrini@inaf.it
² Goethe University Frankfurt, Max-von-Laue-Strasse 1, Frankfurt am Main 60438, Germany
³ INFN – Sezione di Perugia, Via A. Pascoli snc, 06123 Perugia, Italy
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, FI, Italy
⁵ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
⁶ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio Av. 3, 10257 Vilnius, Lithuania
⁷ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁸ IFPU, Institute for the Fundamental Physics of the Universe, Via Beirut 2, 34151 Grignano, Trieste, Italy
⁹ NAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 11 December 2020
Accepted: 9 January 2021

Abstract

Context. Abundance ratios involving Y or other slow-neutron capture elements are routinely used to infer stellar ages.

Aims. We aim to explain the observed [Y/H] and [Y/Mg] abundance ratios of star clusters located in the inner disc with a new prescription for mixing in asymptotic giant branch (AGB) stars.

Methods. In a Galactic chemical evolution model, we adopted a new set of AGB stellar yields in which magnetic mixing was included. We compared the results of the model with a sample of abundances and ages of open clusters located at different Galactocentric distances.

Results. The magnetic mixing causes a less efficient production of Y at high metallicity. A non-negligible fraction of stars with super-solar metallicity is produced in the inner disc, and their Y abundances are affected by the reduced yields. The results of the new AGB model qualitatively reproduce the observed trends for both [Y/H] and [Y/Mg] versus age at different Galactocetric distances.

Conclusions. Our results confirm from a theoretical point of view that the relation between [Y/Mg] and stellar age cannot be ‘universal’, that is, cannot be the same in every part of the Galaxy. It has a strong dependence on the star formation rate, on the s-process yields, and on their relation with metallicity, and it therefore varies throughout the Galactic disc.