Young alpha-enriched giant stars in the solar neighbourhood
  We derive age constraints for 1639 red giants in the APOKASC sample for which
seismic parameters from Kepler, as well as effective temperatures,
metallicities and [alpha/Fe] values from APOGEE DR12 are available. We
investigate the relation between age and chemical abundances for these stars,
using a simple and robust approach to obtain ages. We first derive stellar
masses using standard seismic scaling relations, then determine the maximum
possible age for each star as function of its mass and metallicity,
independently of its evolutionary stage. While the overall trend between
maximum age and chemical abundances is a declining fraction of young stars with
increasing [alpha/Fe], at least 14 out of 241 stars with [alpha/Fe]>0.13 are
younger than 6 Gyr. Five stars with [alpha/Fe]>0.2 have ages below 4 Gyr. We
examine the effect of modifications in the standard seismic scaling relations,
as well as the effect of very low helium fractions, but these changes are not
enough to make these stars as old as usually expected for alpha-rich stars
(i.e., ages greater than 8-9 Gyr). Such unusual alpha-rich young stars have
also been detected by other surveys, but defy simple explanations in a galaxy
evolution context.
