Right round: onset and long-term evolution of rotation in star clusters

Abstract

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 result increases by a factor of $\sim5$ the number of clusters identified as rotators so far and it finally enables an observational reading of cluster rotation as a function of time. Young ($<500$ Myr) clusters show a larger range of rotation velocities than older systems. In addition, at young ages we observe a significantly larger fraction ($50\%-60\%$) of rotating systems than at older ones ($\sim 15\%$). 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 sub-sample 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 an age smaller than their orbital period 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.

Figures (9)

• Figure 1: NGC 869 member properties: the left panel shows the spatial distribution in Galactic coordinates, in the middle panel the vector-point diagram is presented while the right panel displays the distribution in the color-magnitude diagram. The small inset shows the parallax distribution. Gray points are the starting sample of Gaia sources used in the clustering analysis, while in blue are NGC 869 member stars.
• Figure 2: Velocity dispersion (upper panel) and rotation (lower panel) profiles for the OC NGC 869. Blue circles represent the observed values as obtained by using a maximum-likelihood approach on binned data. The black lines and shaded grey areas represent the best-fit profiles obtained from the Bayesian analysis on discrete velocities described in Sect. \ref{['sec:vel_disp']}
• Figure 3: The upper panel show the spatial distribution of members stars of NGC 6871 in Cartesian coordinates with arrows showing the velocity vectors on the plane of the sky. The arrow lengths are proportional to the speed on the plane of the sky while their colors map the radial component ($v_{\rm RAD}$) of the velocity. Positive values point outward. The bottom panel shows the distribution of members in the $v_{\rm RAD}-R$ diagram. The black line represents the bet-fit value for $\langle v_{\rm RAD} \rangle$ and the grey dashed area its uncertainties.
• Figure 4: Distribution of the three velocity components as a function of the position angle for NGC 3532. The black line and the dashed grey area represent the best-fit and relative uncertanties as obtained in Sect. \ref{['sec:3d_analysis']}.
• Figure 5: $a/r_{\rm max}$ and $R_{\rm peak}/r_{\rm max}$ distribution for OCs in the sample. Blue circles represent clusters selected as described in Sect. \ref{['sec:rotation']}.