The vertical metallicity gradients of mono-age stellar populations in
  the Milky Way with the RAVE and Gaia data
  We investigate the vertical metallicity gradients of five mono-age stellar
populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected
from the cross-matched Tycho-Gaia Astrometric Solution (TGAS) and RAdial
Velocity Experiment (RAVE) Data Release 5. We find a correlation between the
vertical metallicity gradients and age, with no vertical metallicity gradient
in the youngest population and an increasingly steeper negative vertical
metallicity gradient for the older stellar populations. The metallicity at disc
plane remains almost constant between 2 and 8 Gyr, and it becomes significantly
lower for the $8 < \tau \leqslant 11$ Gyr population. The current analysis also
reveals that the intrinsic dispersion in metallicity increases steadily with
age. We discuss that our results are consistent with a scenario that (thin)
disc stars formed from a flaring (thin) star-forming disc.
