Gaia and Hubble Unveil the Kinematics of Stellar Populations in the Type II Globular Clusters ω Centauri and M22

Abstract

The origin of multiple stellar populations in globular clusters (GCs) is one of the greatest mysteries of modern stellar astrophysics. N-body simulations suggest that the present-day dynamics of GC stars can constrain the events that occurred at high redshift and led to the formation of multiple populations. Here, we combine multiband photometry from the Hubble Space Telescope (HST) and ground-based facilities with HST and Gaia Data Release 2 proper motions to investigate the spatial distributions and the motions in the plane of the sky of multiple populations in the Type II GCs NGC 5139 (ω Centauri) and NGC 6656 (M22). We first analyzed stellar populations with different metallicities. Fe-poor and Fe-rich stars in M22 share similar spatial distributions and rotation patterns and exhibit similar isotropic motions. Similarly, the two main populations with different iron abundance in ω Centauri share similar ellipticities and rotation patterns. When different radial regions are analyzed, we find that the rotation amplitude decreases from the center toward the external regions. Fe-poor and Fe-rich stars of ω Centauri are radially anisotropic in the central region and show similar degrees of anisotropy. We also investigate the stellar populations with different light-element abundances and find that their N-rich stars exhibit higher ellipticity than N-poor stars. In ω Centauri both stellar groups are radially anisotropic. Interestingly, N-rich, Fe-rich stars exhibit different rotation patterns than N-poor stars with similar metallicities. The stellar populations with different nitrogen of M22 exhibit similar rotation patterns and isotropic motions. We discuss these findings in the context of the formation of multiple populations.

Multiple Populations in Star Clusters

“…Stars with different iron abundances of both ω Centauri and M22 share similar ellipticities, while N-rich stars are more flattened than N-poor stars. This fact is consistent with a scenario where distinct processes are responsible for the enrichment in iron and in p-capture elements s [204].…”

“…• Work based on N-body simulations of GC stars has shown that the frequency of binaries among multiple populations and their 3D kinematics and spatial distributions would provide information on the initial configuration of 1P and 2P stars [198,205,275,276]. Nevertheless, observational constraints are provided for a handful of GCs only [201,204,[209][210][211][212]277]. Moreover, these studies are often limited to the central cluster regions, where the initial configuration of 1P and 2P stars have been erased by the GC dynamical evolution.…”

The role of rotation on the formation of second generation stars in globular clusters

“…2). Moreover, we found that FG and SG stars are rotating in phase, a feature often found in present-day GCs (Milone et al 2018;Cordoni et al 2020b).…”

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XXIII. Proper-motion Catalogs and Internal Kinematics

“…We now know that 1G and 2G stars share similar kinematic features in dynamically old GCs, or at least in the centermost regions of GCs where the two-body relaxation time is short, because two-body processes have already erased any kinematic differences between mPOPs (Anderson & van der Marel 2010;Libralato et al 2018aLibralato et al , 2019. The outermost regions of GCs are instead less relaxed, and some fingerprints of the initial kinematic differences between mPOPs can still be detected (Richer et al 2013;Bellini et al 2015;Bellini et al 2018;Dalessandro et al 2018aDalessandro et al , 2018bMilone et al 2018;Dalessandro et al 2019;Cordoni et al 2020aCordoni et al , 2020b.…”