2026/07/20 by Samrat Biswas, Biman J Medhi, Sergio Messina +9
Physics and Astronomy · #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research
paper · pdf · doi:10.3847/1538-3881/ae8c38
We re-investigate the open cluster UBC 63 using the Gaia DR3 data and show that, rather than being a single cluster as previously classified, it is a compelling candidate for a double cluster undergoing an unbound flyby interaction. A GMM decomposition performed in the 5D astrometric space reveals the two statistically distinct components, namely UBC 63A (98 members, Age = 21 ± 4 Myr) and UBC 63B (148 members, Age = 562 ± 43 Myr). A significant age difference of ΔAge = 541 ± 43 Myr between the clusters, rules out coeval formation. Their 3D separation of 60 ± 29 pc at the birth-epoch of the younger cluster, indicates that the clusters might have originated from the same molecular cloud complex. At present, the system exhibits a 3D separation of 26 ± 8 pc, with a relative velocity of 3.60 ± 1.80 km s-1. Orbital integrations and N-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of 7 ± 2 pc only ∼~6 Myr ago and predict a rapid divergence to a separation of 491 ± 213 pc within the next ∼100 Myr. The low escape velocity (V\rm esc = 0.51 ± 0.12 km s-1) of the system compared to the relative 3D velocity indicates that they are gravitationally unbound. Their low tidal factors, elongated structures and populations extending beyond the Jacobi radii may reflect a strong transient tidal interaction between the clusters.