Investigation of two candidate tidal capture binary star clusters in the Milky Way using Gaia DR3

¹ Shanghai Key Lab for Astrophysics, Shanghai Normal University, Shanghai 200345, China
² Institute of Astronomy and Information, Dali University, Dali 671003, China

^★ Corresponding authors: zhongmuli@126.com; zhangshaohua@shnu.edu.cn

Received: 28 July 2025
Accepted: 8 September 2025

Abstract

Context. In addition to simultaneous formation, tidal capture has been proposed as a significant mechanism for the formation of binary star clusters. Nevertheless, the majority of the candidate captured pairs reported in catalogues are optical pairs or flyby encounters, rather than physically bound binary star clusters. Therefore, verifying this formation mechanism requires further observational data.

Aims. This study investigates the dynamical evolution of two pairs of candidate capture binary star clusters, namely ASCC 71 and ESO 064-05, and NGC 2129 and UBC 437, which were previously identified as tidal capture systems.

Methods. We derived the astrometric and astrophysical parameters of each cluster based on Gaia DR3 data. We traced their past trajectories and predicted their future dynamical evolution using Galactic potential models and N-body simulations.

Results. The Bayesian distance estimates for ASCC 71, ESO 064-05, NGC 2129, and UBC 437 are 1338.85 ± 23.72 pc, 1334.58 ± 18.35 pc, 2090.53 ± 64.71 pc, and 2083.12 ± 70.39 pc, respectively. The ages of ASCC 71 and ESO 064-05 are 136₋₄₀⁺⁹⁵ Myr and 56₋₅⁺¹⁵ Myr, with metal abundances of 0.006_−0.002^+0.002 and 0.010_−0.004^+0.017. The spatial separation between the two is 24.54 ± 16.63 pc, and the difference in radial velocity is 2.06 ± 0.24 km s⁻¹. NGC 2129 and UBC 437 have ages of 46₋₂₀⁺¹¹⁵ Myr and 426_−135+570 Myr, with metal abundance of 0.008_−0.006^+0.022 and 0.006_−0.004^+0.004. The ages of the clusters are very different (approximately 80 Myr and 380 Myr), indicating they have different origins. The opposite signs of the L_x angular momentum components of NGC 2129 and UBC 437 show that the clusters rotate in opposite directions about the X-axis, suggesting that their close proximity results from a chance encounter. By comparing the orbital velocities and relative velocity differences of the two cluster pairs, we find that both are currently gravitationally unbound. In addition, orbital integrations and N-body simulations indicate that both pairs of clusters will rapidly separate in the future.

Conclusions. These two pairs of clusters did not form gravitationally bound binary star clusters in a flyby encounter.