The evolution of the Milky Way's thin disc radial metallicity gradient with K2 asteroseismic ages
The radial metallicity distribution of the Milky Way's disc is an
important observational constraint for models of the formation
and evolution of our Galaxy. It informs our understanding of the
chemical enrichment of the Galactic disc and the dynamical
processes therein, particularly radial migration. We investigate
how the metallicity changes with guiding radius in the thin disc
using a sample of red giant stars with robust astrometric,
spectroscopic, and asteroseismic parameters. Our sample contains
668 stars with guiding radii 4 < R_g < 11 kpc and
asteroseismic ages covering the whole history of the thin disc
with precision ≈ 25 per cent. We use MCMC
analysis to measure the gradient and its intrinsic spread in
bins of age and construct a hierarchical Bayesian model to
investigate the evolution of these parameters independently of
the bins. We find a smooth evolution of the gradient from
≈-0.07 dex kpc^-1 in the youngest stars
to ≈-0.04 dex kpc^-1 in stars older
than 10 Gyr, with no break at intermediate ages. Our results are
consistent with those based on asteroseismic ages from CoRoT,
with that found in Cepheid variables for stars younger than 1
Gyr, and with open clusters for stars younger than 6 Gyr. For
older stars we find a significantly lower metallicity in our
sample than in the clusters, suggesting a survival bias
favouring more metal-rich clusters. We also find that the
chemical evolution model of Chiappini '09 is too metal poor in
the early stages of disc formation. Our results provide strong
new constraints for the growth and enrichment of the thin disc
and radial migration, which will facilitate new tests of model
conditions and physics.