Evolution of the disky second generation of stars in globular clusters on cosmological timescales

¹ Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
² Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
³ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
⁴ Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
⁵ Rudolf Peierls Centre for Theoretical Physics, Parks Road, OX1 3PU, Oxford, UK

^★ Corresponding author; berczik@mao.kiev.ua

Received: 1 November 2024
Accepted: 2 January 2025

Abstract

Context. Many Milky Way globular clusters (GCs) host multiple stellar populations, challenging the traditional view that GCs are single-population systems. It has been suggested that second-generation stars could form in a disk from gas lost by first-generation stars or from external accreted gas. Understanding how these multiple stellar populations evolve under a time-varying Galactic tidal field is crucial for studying internal mixing, the rotational properties, and mass loss of GCs over cosmological timescales.

Aims. We investigated how the introduction of a second stellar generation affects mass loss’ internal mixing, and rotational properties of GCs in a time-varying Galactic tidal field and different orbital configurations.

Methods. We conducted direct N-body simulations of GCs on three types of orbits derived from the observed Milky Way GCs using state-of-the-art stellar evolution prescriptions. We evolved the clusters for 8 Gyr in the time-varying Galactic potential of the IllustrisTNG-100 cosmological simulation. After 2 Gyr, we introduced a second stellar generation, comprising 5% of the initial mass of the first generation, as a flattened disk of stars. For comparison, we ran control simulations using a static Galactic potential and isolated clusters.

Results. We present here the mass loss, structural evolution, and kinematic properties of GCs with two stellar generations, focusing on tidal mass’ half-mass radii, velocity distributions, and angular momentum. We also examine the transition of the second generation from a flattened disk to a spherical shape.

Conclusions. Our results show that the mass loss of GCs depends primarily on their orbital parameters, with tighter orbits leading to higher mass loss. The growth of the Galaxy led to tighter orbits’ implying that the GCs lost much less mass than if the Galaxy had always had its current mass. The initially flattened second-generation disk became nearly spherical within one relaxation time. However, whether its distinct rotational signature was retained depends on the orbit: for the long radial orbit, it vanished quickly; for the tube orbit' it lasted several billion years for the circular orbit' rotation persisted until the present day.