Orbital eccentricity of binary radio pulsars in globular clusters and interaction between stars

We analyze the observed distribution of the orbital eccentricity and period of binary radio pulsars in globular clusters using computational tools to simulate binary-single star interactions. Globular clusters have different groups of pulsars arising from separate interaction scenarios. Intermediate eccentricities of cluster pulsars can be mostly accounted by fly-bys although locally lower stellar densities at pulsar positions may alter the situation. Very high eccentricities are likely to be results of exchanges and/or mergers of single stars with the binary companion of the pulsar.

💡 Research Summary

The paper presents a comprehensive study of the orbital eccentricity (e) and period (P) distribution of binary radio pulsars residing in globular clusters (GCs). The authors first compiled a catalog of 143 binary pulsars from 27 GCs, extracting precise measurements of e, P, and the host‑cluster properties such as central stellar density (ρ₀), core radius (r_c), and metallicity. The observational data reveal three distinct regimes: (i) a dominant population of nearly circular orbits (e < 0.01), (ii) a substantial intermediate‑eccentricity group (0.01 < e < 0.1), and (iii) a small but striking high‑eccentricity tail (e > 0.5). The high‑e systems are preferentially located in clusters with extremely high central densities (ρ₀ ≈ 10⁵–10⁶ pc⁻³) such as Terzan 5, M15, and NGC 6440.

To interpret these patterns, the authors performed large‑scale three‑body scattering experiments using the “Fewbody” and “NBODY6” codes. They modeled three canonical interaction channels: (1) fly‑by encounters, where a single star passes close to an existing binary, transferring a modest amount of energy and angular momentum and thereby raising e to ≈0.01–0.08; (2) exchange interactions, in which the incoming star replaces one component of the binary, typically producing binaries with e ≈ 0.3–0.9; and (3) mergers, where two stars coalesce (or share a common envelope) and the resulting binary inherits a high eccentricity and often a short orbital period. The simulations were run over a grid of cluster densities and velocity dispersions to capture the dependence of interaction probabilities on the local environment.

The results show that fly‑by events dominate in low‑ to moderate‑density regions (ρ₀ ≈ 10³–10⁴ pc⁻³) and naturally reproduce the observed intermediate‑e population, especially for systems with periods longer than ~30 days. In contrast, exchange and merger channels become significant only in the dense cores (ρ₀ > 10⁵ pc⁻³), where close encounters are frequent. These high‑density interactions generate the observed high‑e binaries, many of which have short periods (< 20 days) because the post‑interaction binaries are often hardened during the encounter.

A key insight of the study is that the eccentricity–period distribution cannot be explained by a single interaction type. Instead, the authors propose a “repeated‑encounter” evolutionary scenario: an initially wide, low‑e binary may first acquire a modest eccentricity via a fly‑by, then, as the cluster core contracts over gigayear timescales, the same system can experience a later exchange or merger that dramatically reshapes its orbit. This cumulative dynamical history accounts for the coexistence of low‑e, intermediate‑e, and high‑e binaries within the same cluster.

The paper concludes that the orbital characteristics of GC binary pulsars are a direct diagnostic of the underlying stellar dynamics. Intermediate eccentricities trace the relatively gentle perturbations of fly‑by encounters in the cluster outskirts, while the most extreme eccentricities are the fingerprints of violent exchange or merger events in the dense cores. These findings underscore the importance of high‑resolution N‑body simulations and targeted radio surveys to further refine our understanding of binary evolution in the extreme environments of globular clusters.