Exploring the dynamical evolution of binary stars in multiple-population globular clusters

TL;DR

The paper addresses how binary stars evolve in globular clusters hosting two stellar populations with different initial central concentrations. It employs a suite of MOCCA Monte Carlo simulations to model long-term dynamical evolution, including binary evolution via SSE/BSE and dynamical encounters via FEWBODY, with hardening and disruption governed by the binary binding energy context $x=\frac{E_b}{m\sigma^2}$ (hard binaries satisfy $x>1$). The study finds that the centrally concentrated P2 experiences more frequent encounters, leading to rapid disruption of wide binaries and stronger hardening of compact binaries, producing a central, hard-binary–biased P2 population and steeper radial trends in binary incidence between P1 and P2; binaries, in general, mix more slowly than single stars, preserving memory of their initial MP configuration, while mixed binaries form predominantly in the core through exchange interactions and MS–WD binaries show MP-dependent radial distributions. These results demonstrate that binary demographics can serve as robust tracers of a globular cluster's initial conditions and dynamical history, offering observable predictions that align with MP-related trends seen in Galactic clusters and informing MP formation scenarios.

Abstract

The presence of multiple stellar populations in globular clusters leads to a complex dynamical environment that significantly influences the evolution of binary stars, which in turn impacts the evolution of the cluster itself. For this study, we used a series of Monte Carlo simulations run with the MOCCA code to investigate the long-term dynamical evolution of binary stars in globular clusters hosting two distinct stellar populations. We explored how global binary properties such as incidence, fraction, and spatial distribution evolve over time due to the unique dynamical environment associated with each population. Our results show how binaries in the more centrally concentrated second population (P2) experience increased rates of hardening and disruption relative to the first population (P1), leading to distinct radial profiles in binary incidence and fraction. We also demonstrate the difference in spatial mixing timescales for binaries compared to single stars, where binary stars in each population retain some memory of their initial configurations even after complete single star mixing. Additionally, we investigated the formation and evolution of mixed binaries (binaries composed of a P1 component and a P2 component), which form primarily within the core through dynamical interactions. Finally, we studied main sequence--white dwarf binaries and find that they represent a larger fraction of binaries in P1 compared to P2. The results of this paper highlight the interplay between cluster dynamics and the evolution of binary stars and how binaries can act as tracers of the cluster's initial conditions and dynamical evolution.

Figures (15)

• Figure 1: Distributions of the semimajor axes (top) and hardness values (bottom) of the P1 (blue), P2 (red), and mixed (green) binaries for the mr025c005fb10 simulation evolved to 12 Gyr and normalized such that the total area under each histogram equals 1.
• Figure 2: Time evolution of the global binary incidence within P1 (dashed line), within P2 (dotted line), and within all the stars in the cluster (P1+P2) (solid line). Incidence refers to the fraction of objects within each population that are binaries (see Eq. 3).
• Figure 3: Time evolution of the ratio of the global P1 binary incidence to the global P2 binary incidence, normalized to the current half-mass relaxation time ($t_{rh}(t)$). Incidence refers to the fraction of objects within each population that are binaries (see Eq. 3).
• Figure 4: Radial profile of the incidence of binaries in P1 (blue), P2 (red), and both populations (black) as a function of the projected distance from the cluster center normalized to the projected half-light radius for the mr025c005fb10 simulation evolved to 12 Gyr. We report the median of 100 random realizations of the 2D spatial projection, with the shaded regions representing the 25th and 75th percentiles. Incidence refers to the fraction of objects within each population that are binaries (see Eq. 3).
• Figure 5: Radial profile of the ratio of the incidence of binaries in P1 to the incidence in P2 at 12 Gyr. We report the median of 100 random realizations of the 2D spatial projection, with the shaded regions representing the 25th and 75th percentiles. Incidence refers to the fraction of objects within each population that are binaries (see Eq. 3).