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