The system of open star clusters Collinder 135 and UBC 7

Given the closeness of the two open clusters Collinder 135 and UBC 7 on the sky, we investigated the possibility that the two clusters are physically related. We recovered the present-day stellar membership in these open clusters (300 pc from the Sun) to constrain their kinematic parameters, ages, and masses and to restore their primordial phase space configuration. We investigate the system of these two clusters and possibly coeval stars in 6D space with Gaia EDR3. The actual separation of the cluster centers is equal to 22 pc. Their age is identical, between 40 and 50 Myr, and space motions similar. We conclude that the two clusters Cr 135 and UBC 7 might have formed a physical pair based on the observational evidence as well as numerical simulations. The orbital integration of cluster trajectories backward in time shows that the clusters were much closer in the past if their current line-of-sight velocities are very similar and the total mass is more than seven times larger than the mass of the most reliable members. The census of stars related to this system in the broad surrounding area reveals an extended asymmetric halo of stars related to the system, of total mass comparative to the mass of stars in clusters. This supports the hypothesis of strong cluster mass loss by violent relaxation in the first 20 Myr.