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arXiv · 2607.18224

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

Samrat Biswas·Biman J Medhi·Sergio Messina·Zhanpeng Zhu·Songmei Qin·Sukanta Deb·A. H. Sheikh·H. S Das·Motohide Tamura·Maheshwar Gopinathan·Ram sagar
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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 $Δ\mathrm{Age} = 541 \pm 43$ 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_{\rm 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.

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Samrat Biswas, Biman J Medhi, Sergio Messina, Zhanpeng Zhu, Songmei Qin, Sukanta Deb, A. H. Sheikh, H. S Das, Motohide Tamura, Maheshwar Gopinathan, Ram sagar. 2026-07-20. Discovery of an Unbound Flyby Companion of UBC 63: In the Immediate Aftermath of a Close Encounter. https://doi.org/10.3847/1538-3881%2Fae8c38

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