A JWST project on 47 Tucanae

Binaries among multiple populations

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
³ Department of Astronomy, Indiana University, Bloomington, Swain West, 727 E. 3rd Street, IN 47405, USA
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, Via Frascati 33, 00077 Monte Porzio Catone, Italy
⁶ Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University, Seoul 03722, Korea
⁷ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China
⁸ Department of Physics and Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-Gu, Seoul 05006, Republic of Korea
⁹ Department of Physics, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Dushu Lake Science and Education Innovation District, Suzhou 215123, Jiangsu Province, PR China
¹⁰ Shanghai Key Laboratory for Astrophysics, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, PR China

^★ Corresponding author: antonino.milone@unipd.it

Received: 6 September 2024
Accepted: 24 March 2025

Abstract

Almost all globular clusters (GCs) contain multiple stellar populations consisting of stars with varying helium and light-element abundances. These populations include first-population stars, which exhibit similar chemical compositions as halo-field stars with comparable [Fe/H], and second-population stars, characterized by higher helium and nitrogen abundances along with reduced levels of oxygen and carbon. Nowadays, one of the most intriguing open questions about GCs pertains to the formation and evolution of their multiple populations. Recent works based on N-body simulations of GCs show that the fractions and characteristics of binary stars can serve as dynamic indicators of the formation period of multiple-population GCs and their subsequent dynamical evolution. Nevertheless, the incidence of binaries among multiple populations is still poorly studied. Moreover, the few available observational studies focus only on the bright stars of a few GCs. We used deep images of the GC 47 Tucanae collected with the James Webb and the Hubble space telescopes to investigate the incidence of binaries among multiple populations of M dwarfs and bright main- sequence stars. To reach this objective, we used UV, optical, and near-infrared filters to construct photometric diagrams that allowed us to disentangle binary systems and multiple populations. Moreover, we compared these observations with a large sample of simulated binaries. In the cluster central regions, the incidence of binaries among first-population stars is only slightly higher than that of second- population stars. In contrast, in the external regions, the majority of the studied binaries (≳85%) are composed of first-population stars. These results are consistent with the GC formation scenarios in which the second-population stars originate in the cluster’s central region, forming a compact and dense stellar group within a more extended system of first-population stars.