Exploring the formation environment and dynamics of multiple stellar populations in globular clusters through binary systems

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
² Department of Astronomy, Indiana University, Swain West, 727 E. 3rd Street, Bloomington, IN 47405, USA
³ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122. Italy
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China

^★ Corresponding author: emanuele.bortolan@studenti.unipd.it

Received: 28 October 2024
Accepted: 9 March 2025

Abstract

Aims. Globular clusters (GCs) are known to host distinct stellar populations, characterized by different chemical compositions. Despite extensive research, the origin of these populations remains elusive. According to many formation scenarios, the second population (2P) originated within a compact and denser region embedded in a more extended first population (1P) system. As a result, 2P binaries should be disrupted at a larger rate than 1P binaries. For this reason, binary systems offer valuable insight into the environments in which these stellar populations formed and evolved.

Methods. We analyzed the fraction of binaries among 1P and 2P M dwarfs in the outer region of NGC 288 using Hubble Space Telescope data. We combined our results with those from a previous work, where we inferred the fraction of 1P and 2P binaries in the cluster center.

Results. In the outer region, we find a predominance of 1P binaries (97₋₃⁺¹ %) compared to 2P binaries (3 ± 1%) corresponding to an incidence of binaries with a mass ratio (i.e., the ratio between the masses of the primary and secondary star) greater than 0.5 equal to 6.4 ± 1.7% for the 1P and 0.3 ± 0.2% for the 2P. These binary fractions and incidences differ from those found in the cluster’s central region, where the 1P and 2P exhibit similar binary incidences and fractions. These results are in general agreement with the predictions of simulations following the evolution of binary stars in multiple-population GCs, starting with a dense 2P subsystem concentrated in the central regions of a 1P system.