Bimodal regime in young massive clusters leading to subsequent stellar generations

TL;DR

This paper reviews the bimodal regime in young massive clusters where hot gas becomes thermally unstable, leading to dense clump formation and subsequent second-generation star formation enriched by first-generation stellar products.

Contribution

It discusses the conditions and implications of the bimodal regime for star formation in massive clusters, highlighting the process of gas cooling, clump formation, and secondary star formation.

Findings

Hot gas in dense clusters becomes thermally unstable.

Dense clumps form and are trapped by gravity.

Second-generation stars form from accumulated, enriched gas.

Abstract

Massive stars in young massive clusters insert tremendous amounts of mass and energy into their surroundings in the form of stellar winds and supernova ejecta. Mutual shock-shock collisions lead to formation of hot gas, filling the volume of the cluster. The pressure of this gas then drives a powerful cluster wind. However, it has been shown that if the cluster is massive and dense enough, it can evolve in the so--called bimodal regime, in which the hot gas inside the cluster becomes thermally unstable and forms dense clumps which are trapped inside the cluster by its gravity. We will review works on the bimodal regime and discuss the implications for the formation of subsequent stellar generations. The mass accumulates inside the cluster and as soon as a high enough column density is reached, the interior of the clumps becomes self-shielded against the ionising radiation of stars and the clumps collapse and form new stars. The second stellar generation will be enriched by products of stellar evolution from the first generation, and will be concentrated near the cluster center.