Reaction of Massive Clusters to Gas Expulsion - The cluster density dependence

TL;DR

This study investigates how stellar interactions influence mass loss in massive star clusters after gas expulsion, revealing that encounters significantly affect cluster evolution, especially in dense environments, with implications for understanding cluster dynamics.

Contribution

The paper demonstrates that stellar interactions cause notable mass loss in massive clusters, emphasizing the importance of realistic stellar mass distributions in modeling cluster evolution.

Findings

Stellar interactions cause up to 20% mass loss in dense clusters.

Even in loosely bound associations, interactions lead to at least 5% mass loss.

Encounter-induced mass loss depends on cluster density, with a shallower density dependence than previously thought.

Abstract

The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group as long as stellar interactions can be neglected. We investigate under which circumstances stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different SFEs. Concentrating on the most massive clusters observed in the Milky Way, we test to what extend the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size.We find that stellar interactions are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way, making ejections the prime mass loss process in such systems. Even in the loosely bound OB associations stellar interactions are responsible for at least ~5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density-dependence of the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.