Close Stellar Encounters in Young, Substructured, Dissolving Star Clusters: Statistics and Effects on Planetary Systems

TL;DR

This study uses N-body simulations to analyze how early close stellar encounters in dissolving, substructured clusters affect planetary systems, finding that such encounters are relatively rare and unlikely to cause significant gravitational stripping.

Contribution

It provides the first detailed quantification of encounter rates and orbital distributions in dissolving, substructured clusters, highlighting the transient nature of encounter rate boosts.

Findings

Encounter rates are higher in substructured clusters compared to relaxed ones.

The boost in encounter rate lasts only about a dynamical time.

Most star-forming environments have low enough encounter rates to prevent significant gravitational stripping.

Abstract

Both simulations and observations indicate that stars form in filamentary, hierarchically clustered associations, most of which disperse into their galactic field once feedback destroys their parent clouds. However, during their early evolution in these substructured environments, stars can undergo close encounters with one another that might have significant impacts on their protoplanetary disks or young planetary systems. We perform N-body simulations of the early evolution of dissolving, substructured clusters with a wide range of properties, with the aim of quantifying the expected number and orbital element distributions of encounters as a function of cluster properties. We show that the presence of substructure both boosts the encounter rate and modifies the distribution of encounter velocities compared to what would be expected for a dynamically relaxed cluster. However, the boost only lasts for a dynamical time, and as a result the overall number of encounters expected remains low enough that gravitational stripping is unlikely to be a significant effect for the vast majority of star-forming environments in the Galaxy. We briefly discuss the implications of this result for models of the origin of the Solar System, and of free-floating planets. We also provide tabulated encounter rates and orbital element distributions suitable for inclusion in population synthesis models of planet formation in a clustered environment.