Open cluster kinematics with Gaia DR2
  Context. Open clusters are very good tracers of the evolution of the Galactic
disc. Thanks to Gaia, their kinematics can be investigated with an
unprecedented precision and accuracy. Aims. The distribution of open clusters
in the 6D phase space is revisited with Gaia DR2. Methods. The weighted mean
radial velocity of open clusters was determined, using the most probable
members available from a previous astrometric investigation that also provided
mean parallaxes and proper motions. Those parameters, all derived from Gaia DR2
only, were combined to provide the 6D phase space information of 861 clusters.
The velocity distribution of nearby clusters was investigated, as well as the
spatial and velocity distributions of the whole sample as a function of age. A
high quality subsample was used to investigate some possible pairs and groups
of clusters sharing the same Galactic position and velocity. Results. For the
high quality sample that has 406 clusters, the median uncertainty of the
weighted mean radial velocity is 0.5 km/s. The accuracy, assessed by comparison
to ground-based high resolution spectroscopy, is better than 1 km/s. Open
clusters nicely follow the velocity distribution of field stars in the close
Solar neighbourhood previously revealed by Gaia DR2. As expected, the vertical
distribution of young clusters is very flat but the novelty is the high
precision to which this can be seen. The dispersion of vertical velocities of
young clusters is at the level of 5 km/s. Clusters older than 1 Gyr span
distances to the Galactic plane up to 1 kpc with a vertical velocity dispersion
of 14 km/s, typical of the thin disc. Five pairs of clusters and one group with
five members are possibly physically related. Other binary candidates
previously identified turn out to be chance alignment.
