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Abstract Our understanding of the kinematic properties of multiple stellar populations (mPOPs) in Galactic globular clusters (GCs) is still limited compared to what we know about their chemical and photometric characteristics. Such limitation arises from the lack of a comprehensive observational investigation of this topic. Here we present the first homogeneous kinematic analysis of mPOPs in 56 GCs based on high-precision proper motions computed with Hubble Space Telescope data. We focused on red-giant-branch stars, for which the mPOP tagging is clearer, and measured the velocity dispersion of stars belonging to first (1G) and second generations (2G). We find that 1G stars are generally kinematically isotropic even at the half-light radius, whereas 2G stars are isotropic at the center and become radially anisotropic before the half-light radius. The radial anisotropy is induced by a lower tangential velocity dispersion of 2G stars with respect to the 1G population, while the radial component of the motion is comparable. We also show possible evidence that the kinematic properties of mPOPs are affected by the Galactic tidal field, corroborating previous observational and theoretical results suggesting a relation between the strength of the external tidal field and some properties of mPOPs. Although limited to the GCs’ central regions, our analysis leads to new insights into the mPOP phenomenon, and provides the motivation for future observational studies of the internal kinematics of mPOPs. 

The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with${72}_{-12}^{+10}$% of its stars strongly enhanced inr-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color–magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization, forming ∼80% of the stars in the galaxy, while the remainder of the stars formed ∼3 Gyr later. When the bursts are allowed to have nonzero durations, we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 ± 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence ofr-process-enhanced stars demonstrates that ther-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and ther-process nucleosynthesis to be less than 500 Myr. This measurement rules out anr-process source with a delay time of several Gyr or more, such as GW170817.