The structural properties of multiple populations in globular clusters: The instructive case of NGC 3201

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
e-mail: mario.cadelano@unibo.it
² INAF-Osservatorio di Astrofisica e Scienze dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Department of Astronomy, Indiana University, Swain West, 727 E. 3rd Street, Bloomington, IN 47405, USA

Received: 19 December 2023
Accepted: 14 February 2024

Abstract

All multiple population (MP) formation models in globular clusters (GCs) predict that second population (SP) stars form more centrally concentrated than the first population (FP). As dynamical evolution proceeds, spatial differences are progressively erased and only dynamically young clusters are expected to retain a partial memory of the initial structural differences. In recent years, this picture has been supported by observations of the MP radial distributions of both Galactic and extragalactic GCs. However, more recent observations have suggested that in some systems, FPs might actually form more centrally segregated, with NGC 3201 being one significant example of such a possibility. Here, we present a detailed morphological and kinematic characterization of the MPs in NGC 3201, based on a combination of photometric and astrometric data. We show that the distribution of the SP is clearly bimodal. Specifically, the SP is significantly more centrally concentrated than the FP within ∼1.3 cluster’s half-mass radius. Beyond this point, the SP fraction increases again, likely due to asymmetries in the spatial distributions of the two populations. The central concentration of the SP observed in the central regions implies that it formed more centrally concentrated than the FP, even more so than what is observed in the present-day. This interpretation is supported by the key information provided by the MP kinematic properties. Indeed, we find that the FP is isotropic across all the sampled cluster extension, while the velocity distribution of the SP becomes radially anisotropic in the cluster’s outer regions, as expected for the dynamical evolution of SP stars formed more centrally concentrated than the FP. The combination of spatial and kinematic observations provide key insights into the dynamical properties of this cluster and lend further support to scenarios in which the SP forms more centrally concentrated than the FP.