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

Abstract

The use of 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. In a Galactic chemical evolution model, we adopt a new set of AGB stellar yields in which magnetic mixing is included. We compare the results of the model with a sample of abundances and ages of open clusters located at different Galactocentric distances. 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] vs age at different Galactocetric distances. Our results confirm from a theoretical point of view that the relationship between [Y/Mg] and stellar age cannot be universal, i.e., the same in every part of the Galaxy. It has a strong dependence on the star formation rate, on the s-process yields and their relation with metallicity, and thus it varies across the Galactic disc.