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

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.

💡 Research Summary

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This paper investigates how the presence of two stellar populations (P1 and P2) in globular clusters influences the long‑term dynamical evolution of binary stars. Using the Monte‑Carlo Cluster Simulator (MOCCA), the authors performed a suite of simulations containing one million stars each, with initial binary fractions of 5 % or 10 % and with P2 comprising either 10 % or 25 % of the total objects. The key structural parameter is the concentration ratio r_P2h / r_P1h, set to 0.05 or 0.10, which makes the second population centrally concentrated relative to the more extended first population. All models start with the same metallicity (Z = 0.001) and a Kroupa IMF, and binaries are generated with a uniform mass‑ratio distribution for massive primaries and random pairing otherwise; semi‑major axes follow a log‑uniform distribution between a contact limit and 100 AU.

The simulations are evolved for 12 Gyr, allowing the authors to trace the evolution of binary hardness, semi‑major‑axis distributions, global binary incidence (the fraction of objects that are binaries), and radial profiles for each population. The main findings are:

1. Hardening and Disruption: Because P2 resides in a denser core with higher velocity dispersion, soft binaries (hardness x < 1) are rapidly disrupted, while hard binaries (x > 1) experience accelerated hardening. P1, occupying the lower‑density outskirts, retains a larger fraction of wide (soft) binaries. Consequently, at 12 Gyr the P2 binary population is dominated by very hard, compact systems, whereas P1 still contains many wider pairs.

2. Binary Incidence Evolution: Initially identical, the binary incidence in P2 declines sharply, whereas P1 shows a more gradual decrease. The ratio I_P1 / I_P2 therefore rises from ~1 to values between 1 and 1.2 during the first few half‑mass relaxation times, flattening once the two populations have mixed spatially. In models subjected to a stronger tidal field (smaller tidal radius), the ratio even declines at late times because the remaining hard P2 binaries survive while many soft P1 binaries that have migrated inward are destroyed.

3. Radial Profiles and Spatial Mixing: Projected radial profiles of binary incidence reveal that P2 binaries are centrally concentrated, with incidence dropping steeply with radius. P1 binaries show a more modest decline and a slight upturn in the outermost regions, reflecting the longer relaxation times and lower encounter rates there. The incidence ratio P1/P2 increases outward, making the cluster outskirts the most diagnostic region for distinguishing the two populations. Mixing of single stars occurs on a timescale of a few half‑mass relaxation times, but binaries retain memory of their original population for longer, especially in the outer halo.

4. Mixed Binaries (P1–P2): Dynamical three‑body encounters in the core produce binaries composed of one star from each population. These mixed binaries are predominantly hard and become more common as the cluster evolves, providing a unique tracer of multi‑population dynamics that would not exist in single‑population clusters.

5. Main‑Sequence–White‑Dwarf Binaries: The fraction of MS–WD binaries is higher in P1 than in P2. The dense core of P2 leads to frequent interactions that either destroy such binaries or prevent their formation, whereas the more benign environment of P1 allows them to survive.

6. Effect of Tidal Field: Simulations with a smaller initial tidal radius (38 pc) experience stronger tidal stripping, which preferentially removes outer‑region stars and flattens the radial incidence profiles. This mimics an older dynamical age and more advanced spatial mixing.

Overall, the study demonstrates that the initial structural differences between stellar populations imprint long‑lasting signatures on the binary population. Binary hardness, survival rate, and spatial distribution act as sensitive diagnostics of the cluster’s initial conditions, its dynamical age, and the strength of the external tidal field. Observational programs that measure binary fractions and radial profiles for different stellar populations can therefore use binaries as powerful tracers of globular‑cluster formation and evolution.