The Hubble Space Telescope UV Legacy Survey of galactic globular clusters. VI. The internal kinematics of the multiple stellar populations in NGC 2808

TL;DR

This study uses high-precision Hubble Space Telescope proper-motion data to analyze the internal kinematics of multiple stellar populations in NGC 2808, revealing differences in velocity anisotropy and dispersion linked to their formation history.

Contribution

It provides the first detailed kinematic analysis of multiple populations in NGC 2808, linking observed dynamics to their initial spatial distributions and evolutionary processes.

Findings

Populations D and E show radially anisotropic velocity distributions.

Stars D and E have smaller tangential velocity dispersions than populations A, B, and C.

Kinematic differences are consistent with diffusion of centrally concentrated populations outward.

Abstract

Numerous observational studies have revealed the ubiquitous presence of multiple stellar populations in globular clusters and cast many hard challenges for the study of the formation and dynamical history of these stellar systems. In this Letter we present the results of a study of the kinematic properties of multiple populations in NGC 2808 based on high-precision Hubble Space Telescope proper-motion measurements. In a recent study, Milone et al. have identified five distinct populations (A, B, C, D, and E) in NGC 2808. Populations D and E coincide with the helium-enhanced populations in the middle and the blue main sequences (mMS and bMS) previously discovered by Piotto et al.; populations A, B, and C correspond to the redder main sequence (rMS) that in the Piotto et al. was associated with the primordial stellar population. Our analysis shows that, in the outermost regions probed (between about 1.5 and 2 times the cluster half-light radius), the velocity distribution of populations D and E is radially anisotropic (the deviation from an isotropic distribution is significant at the ~3.5-sigma level). Stars of populations D and E have a smaller tangential velocity dispersion than those of populations A, B, and C, while no significant differences are found in the radial-velocity dispersion. We present the results of a numerical simulation showing that the observed differences between the kinematics of these stellar populations are consistent with the expected kinematic fingerprint of the diffusion towards the cluster outer regions of stellar populations initially more centrally concentrated.