Kinematics in young star clusters & associations

We use a sample of 28 clusters and associations with ages from ~1-5Myr, where lists of members are available from previous X-ray, optical, and infrared studies. Proper motions from Gaia DR2 reveal 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 ~0.5km/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 {sigma}_1D_=1 to 3km/s, imply that most of the stellar systems in our sample are supervirial and that some are unbound.