Discovery of an Unbound Flyby Companion of UBC 63: In the Immediate Aftermath of a Close Encounter

Abstract

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 $\pm$ 4 Myr) and UBC 63B (148 members, Age = 562 $\pm$ 43 Myr). A significant age difference of $\Delta \mathrm{Age}=541\pm 43$ Myr between the clusters, rules out coeval formation. Their 3D separation of $60\pm 29$ 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 $26\pm 8$ pc, with a relative velocity of $3.60\pm 1.80$ km s${}^{-1}$. Orbital integrations and \textit{N}-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of $7\pm 2$ pc only $\sim$~6 Myr ago and predict a rapid divergence to a separation of $491\pm 213$ pc within the next $\sim$100 Myr. The low escape velocity ($V_{\mathrm{esc}}=0.51\pm 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.

10 pages, 11 figures, 2 tables; Accepted for publication in The Astronomical Journal