The Flattening Metallicity Gradient in the Milky Way's Thin Disk
  We calculate the ages, orbits and phase-space coordinates for a sample of
$\sim$4 million LAMOST and Gaia stars. The ages are crossmatched and compared
with values from two other popular age catalogs which derive the ages using
different methods. Of these $\sim$4 million stars, we select a sample of 1.3
million stars and investigate their radial metallicity gradients (as determined
by orbital radii) as a function of their ages. This analysis is performed on
various subsets of the data split by chemistry and orbital parameters. We find
that commonly used selections for ``thin disk'' stars (such as low-$\alpha$
chemistry or vertically thin orbits) yield radial metallicity gradients which
generally grow shallower for the oldest stars. We interpret this as a hallmark
feature of radial migration (churning). Constraining our sample to very small
orbital Z$_{max}$ (the maximal height of a star's integrated orbit) makes this
trend most pronounced. A chemistry-based ``thin disk'' selection of
$\alpha$-poor stars displays the same trend, but to a lesser extent.
Intruigingly, we find that ``thick disk'' selections in chemistry and Z$_{max}$
reveal a slightly positive radial metallicity gradients which seem similar in
magnitude at all ages. This may imply that the thick disk population is well
mixed in age, but not in radius. This finding could help constrain conditions
during the early epochs of Milky Way formation, and shed light on processes
such as the accretion and reaccretion of gasses of different metallicities.
