On the Encounter Rate of Open Star Clusters

Probable past and future close encounters of open clusters with known characteristics over 64 million years were calculated by integrating the orbits of cluster centers in the Galactic potential using the galpy package. It is shown that in the Galactic neighborhood of the Sun, pairwise cluster encounters at distances comparable to or smaller than their sizes occur at a characteristic rate of 35–40 events per 1 Myr. Close encounters between open clusters with a significant age difference occur at a rate of 15 events per Myr. It can be expected that in the Galaxy as a whole, such events occur an order of magnitude more frequently per unit time. Thus, dynamical interactions between stellar ensembles of different ages may not be too rare and could influence the properties of stellar populations. A pair of clusters with similar ages — HSC 1428 and Gulliver 22 — was identified as a likely physically bound binary cluster system. A forecast of expected close encounters over the next 32 Myr is provided for 490 pairs of clusters. Currently, 29 pairs of clusters are at their closest approach. The work is partially based on a talk presented at the Modern Stellar Astronomy 2024 conference.