Interrupted Stellar Encounters in Star Clusters

TL;DR

This paper investigates the validity of the isolated encounter assumption in star cluster dynamics by analyzing encounter durations and interruption probabilities, revealing that many encounters are actually influenced by additional stars, especially in lower-mass clusters.

Contribution

It provides a comprehensive analysis of encounter interruptions in star clusters, challenging the common assumption of isolated encounters and highlighting the need to consider complex multi-body interactions.

Findings

20-40% of encounters in low-mass clusters are interrupted by interlopers.

Less than 1% of encounters in globular clusters are interrupted.

Many encounters develop into complex mini-clusters, affecting cluster evolution models.

Abstract

Strong encounters between single stars and binaries play a pivotal role in the evolution of star clusters. Such encounters can also dramatically modify the orbital parameters of binaries, exchange partners in and out of binaries, and are a primary contributor to the rate of physical stellar collisions in star clusters. Often, these encounters are studied under the approximation that they happen quickly enough and within a small enough volume to be considered isolated from the rest of the cluster. In this paper, we study the validity of this assumption through the analysis of a large grid of single - binary and binary - binary scattering experiments. For each encounter we evaluate the encounter duration, and compare this with the expected time until another single or binary star will join the encounter. We find that for lower-mass clusters, similar to typical open clusters in our Galaxy, the percent of encounters that will be "interrupted" by an interloping star or binary may be 20-40% (or higher) in the core, though for typical globular clusters we expect <1% of encounters to be interrupted. Thus, the assumption that strong encounters occur in relative isolation breaks down for certain clusters. Instead, many strong encounters develop into more complex "mini-clusters", which must be accounted for in studying, for example, the internal dynamics of star clusters, and the physical stellar collision rate.