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We present the results of a detailed kinematic analysis of a significant fraction of the known population of Galactic star clusters aimed at constraining the physical mechanisms driving the onset and evolution of cluster rotation. Our study reveals, for the very first time, the presence of rotation in clusters at any age, with about 25%-30% of systems in the sample showing significant evidence of rotation. This increases by a factor of ∼5 the number of clusters identified as rotators so far and finally enables an observational reading of cluster rotation as a function of time. Young clusters (< 500 Myr) show a larger range of rotation velocities than older systems. In addition, 50%-60% of young systems are observed to be significantly rotating, while this fraction drops to ∼15% for the older ones. These purely empirical results are compatible with rotation being imprinted during the very early stages of cluster formation and early evolution and then being progressively erased by the long-term effects of dynamical evolution. For the subsample of clusters for which we were able to perform a full 3D analysis, we calculated the angle between the internal rotation axis and that of the cluster orbital motion. Interestingly, while for clusters with ages that are shorter than their orbital periods we observe similar fractions of prograde and retrograde systems, more evolved clusters appear to be preferentially prograde. We argue that such a behavior is in qualitative agreement with the expectations for the evolution of systems in which primordial rotation was imprinted by the parent molecular cloud and/or by the following hierarchical cluster assembly processes, and in which internal cluster dynamics and interactions with the Galactic field have induced a torque-driven alignment between cluster rotation and orbital motion.
Dalessandro et al. (Fri,) studied this question.