Kinematics in Young Star Clusters and Associations with Gaia DR2
  The Gaia mission has opened a new window into the internal kinematics of
young star clusters at the sub-km/s level, with implications for our
understanding of how star clusters form and evolve. We use a sample of 28
clusters and associations with ages from 1-5 Myr, where lists of members are
available from previous X-ray, optical, and infrared studies. Proper motions
from Gaia DR2 reveals that at least 75% of these systems are expanding;
however, rotation is only detected in one system. Typical expansion velocities
are on the order of ~0.5 km/s, and, in several systems, there is a positive
radial gradient in expansion velocity. Systems that are still embedded in
molecular clouds are less likely to be expanding than those that are partially
or fully revealed. One-dimensional velocity dispersions, which range from 1 to
3 km/s, imply that most of the stellar systems in our sample are supervirial
and that some are unbound. In star-forming regions that contain multiple
clusters or subclusters, we find no evidence that these groups are coalescing,
implying that hierarchical cluster assembly, if it occurs, must happen rapidly
during the embedded stage.
