Linking the rotation of a cluster to the spins of its stars: The kinematics of NGC 6791 and NGC 6819 in 3D

The physics governing the formation of star clusters is still not entirely understood. One open question concerns the amount of angular momentum that newly formed clusters possess after emerging from their parent gas clouds. Recent results suggest an alignment of stellar spins and binary orbital spins in star clusters, which support a scenario in which clusters are born with net angular momentum cascading down to stellar scales. In this paper, we combine Gaia data and published line of sight velocities to explore if NGC 6791 and NGC 6819, two of the clusters for which an alignment of stellar spins has been reported, rotate in the same plane as their stars.We find evidence for rotation in NGC 6791 using both proper motions and line of sight velocities. Our estimate of the inclination angle is broadly consistent with the mean inclination that has been determined for its stars, but the uncertainties are still substantial. Our results identify NGC 6791 as a promising follow-up candidate to investigate the link between cluster and stellar rotation. We find no evidence for rotation in NGC 6819