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

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.