Initial Binary Fraction Drives Dynamical Evolution and Structure of Open Clusters

A numerical simulation study has revealed that the initial binary fraction of open clusters exerts a clear influence on both their dynamical and stellar evolutionary properties over 200 million years. Using direct N-body simulations, researchers demonstrated that primordial binary systems play a central role in driving the structural development and internal kinematics of open star clusters.

To assess how varying binary proportions alter cluster development, researchers employed the direct N-body code NbodyCP to evaluate open cluster models with initial binary fractions of 10 percent, 30 percent, and 50 percent. For each model, ten independent random realisations were executed using different random seeds to quantify stochastic variations tied to initial conditions, while maintaining all global model parameters unchanged.

The kinematic results showed distinct variations across the different binary populations. Models starting with higher initial binary fractions developed broader relative velocity-component distributions and systematically exhibited larger velocity dispersions among their constituent stars.

The structural evolution of the simulated clusters also diverged significantly over the 200 Myr timeline. While the tidal radius decreased across all three models, the model initialized with a 50 percent binary fraction displayed the largest relative reduction. Furthermore, core-radius tracking indicated that the 30 percent and 50 percent models became more compact and displayed smaller large-scale fluctuations than the 10 percent model during later evolutionary stages.

Hertzsprung–Russell diagrams generated from the simulations indicated modest differences in the evolved stellar populations among the three models at later evolutionary stages. Despite these modest differences in evolved stars, normalised stellar-population fractions remained broadly comparable.

The research builds on earlier findings establishing that binary systems represent a key factor influencing the dynamical evolution of open clusters. Prior studies had demonstrated that binary populations are closely related to the dynamical evolutionary stages of young open clusters.

Subject of Research: Effects of initial binary fraction on open cluster dynamical evolution

Article Title: Effects of initial binary fraction on dynamical evolution of open clusters

Article References: Yi, X., Li, Z., Li, M., Zheng, X., Yu, C., Qian, B., & Lan, J. (2026). Effects of initial binary fraction on dynamical evolution of open clusters. Astrophysics and Space Science, 371(11), Article 133. https://doi.org/10.1007/s10509-026-04662-5

Image Credits: AI Generated

DOI: 10.1007/s10509-026-04662-5

Keywords: open clusters, binary stars, stellar dynamics, N-body simulations, stellar evolution, velocity dispersion, tidal radius, core radius, Effects, initial, binary, fraction