The Galactic metallicity gradient shown by open clusters in the light of
  radial migration
  During the last years and decades several individual studies and large-scale
spectroscopic surveys significantly improved our knowledge of the Galactic
metallicity distribution based on open clusters. The availability of Gaia data
provided a further step forward in our knowledge. However, still some open
issues remain, for example the influence of radial migration on the
interpretation of the observed gradients. We used spectroscopic metallicities
from individual studies and from the APOGEE survey to compile a sample of 136
open clusters, with a membership verification based on Gaia DR2. Additionally,
we present photometric metallicity estimates of 14 open clusters in a somewhat
outer Galactic region. Eight age groups allow us to study the evolution of the
metallicity gradient in detail, showing within the errors an almost constant
gradient of about $-$0.06 dex kpc$^{-1}$. Furthermore, using the derived
gradients and an analysis of the individual objects, we estimate a mean
migration rate of 1 kpc Gyr$^{-1}$ for objects up to about 2 Gyr. Here, the
change of the guiding radius is clearly the main contributor. For older and
dynamically hotter objects up to 6 Gyr we infer a lower migration rate of up to
0.5 kpc Gyr$^{-1}$. The influence of epicyclic excursions increases with age
and contributes already about 1 kpc to the total migration distance after 6
Gyr. A comparison of our results with available models shows good agreement.
However, there is still a lack of a suitable coverage of older objects, future
studies are still needed to provide a better sampling in this respect.
