N-body simulations of close encounters between open star clusters

¹ Shanghai Key Lab for Astrophysics, Shanghai Normal University, Shanghai 200345, China
² Institute of Astronomy and Information, Dali University, Dali 671003, China

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 January 2026
Accepted: 27 February 2026

Abstract

Context. In the Milky Way, close encounters between open clusters (OCs) of non-common origin are not rare. However, the dynamical processes governing close encounters between OCs remain poorly understood and may differ significantly from those observed in globular clusters.

Aims. We ran simulations to investigate the interaction mechanisms at play during close encounters between OCs, identify the key structural parameters governing mergers and disruptions, and examine the properties of the resulting merger remnants.

Methods. We performed N-body simulations of OC encounters under a variety of initial conditions, considering clusters with different mass and density ratios as well as different initial orbital parameters.

Results. Across all models, significant dynamical interactions, such as merging and tidal disruption, occur exclusively in the parabolic cases. When two clusters follow parabolic orbits and experience substantial overlap at pericentre, merging or tidal disruption can occur. Cluster pairs with different mass ratios can follow entirely different evolutionary scenarios. In the equal-mass merger cases, during the interaction a fraction of the relative orbital energy is converted into the cluster’s internal energy, driving the system into a gravitationally bound state and ultimately leading to a merger. In unequal-mass merger cases, the merger process more closely resembles the tidal disruption of the companion cluster followed by its accretion, differing from the merging pathway observed in equal-mass cases. Nevertheless, the merger remnants all exhibit significant rotation. The inner regions rotate approximately as a rigid body, while the outer parts also show a rising rotational profile, which is markedly different from that of an isolated rotating cluster.