The AMBRE Project: Solar neighbourhood chemodynamical constraints on
  Galactic disc evolution
  We analysed the chemodynamical evolution of the Galactic disc using precise
[Mg/Fe] abundances from a previous study and accurate Gaia data. For this
purpose, we estimated ages and dynamical properties for 366 MSTO solar
neighbourhood stars from the AMBRE Project using PARSEC isochrones together
with astrometric and photometric values from Gaia DR2. We find a radial
gradient of -0.099 ${\pm}$ 0.031 dex kpc$^{-1}$ for [M/H] and +0.023 ${\pm}$
0.009 dex kpc for the [Mg/Fe] abundance. The steeper [Mg/Fe] gradient than that
found in the literature is a result of the improvement of the AMBRE [Mg/Fe]
estimates in the metal-rich regime. In addition, we find a significant spread
of stellar age at any given [Mg/Fe] value, and observe a clear correlated
dispersion of the [Mg/Fe] abundance with metallicity at a given age. While for
[M/H] < -0.2, a clear age-[Mg/Fe] trend is observed, more metal-rich stars
display ages from 3 up to 12 Gyr, describing an almost flat trend in the
[Mg/Fe]-age relation. Moreover, we report the presence of radially migrated
stars for a wide range of stellar ages, although we note the large
uncertainties of the amplitude of the inferred change in orbital guiding radii.
Finally, we observe the appearance of a second chemical sequence in the outer
disc, 10-12 Gyr ago, populating the metal-poor, low-[Mg/Fe] tail. These stars
are more metal-poor than the coexisting stellar population in the inner parts
of the disc, and show lower [Mg/Fe] abundances than prior disc stars of the
same metallicity, leading to a chemical discontinuity. Our data favour the
rapid formation of an early disc that settled in the inner regions, followed by
the accretion of external metal-poor gas -- probably related to a major
accretion event such as the Gaia-Enceladus/Sausage one -- that may have
triggered the formation of the thin disc population and steepened the abundance
gradient in the early disc.
