Discovery of an unbound flyby companion of UBC 63: In the immediate aftermath of a close encounter

Abstract

We reinvestigate the open cluster UBC 63 using the Gaia Data Release 3 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 Gaussian mixture model 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 ∆Age = 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, hints at a possible common environment origin . 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 (Vesc = 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.