Kinematics in Young Star Clusters and Associations with Gaia DR2

TL;DR

This study uses Gaia DR2 data to analyze the internal kinematics of 28 young star clusters and associations, revealing prevalent expansion, low rotation, and supervirial states, with implications for cluster formation and evolution.

Contribution

It provides the first comprehensive kinematic analysis of young clusters using Gaia DR2, highlighting expansion patterns and the unlikelihood of hierarchical assembly during the early stages.

Findings

75% of systems are expanding

Typical expansion velocity ~0.5 km/s

Most systems are supervirial and possibly unbound

Abstract

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.