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