The Physics of Star Cluster Formation and Evolution

Abstract

Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.

The Molecular Cloud Lifecycle

“…While multiple populations have also been observed in many globular clusters (see Adamo et al 2020;Krause et al 2020), the above recollapse scenario cannot explain this observation, because globular clusters do not exhibit the [Fe/H] spread that would be expected for the chemical enrichment by the type II SNe occurring over multiple collapse cycles. Generally speaking, the competition between feedback and gravity in one-dimensional models can have no other outcome than radial expansion or radial (re)collapse.…”

“…3 below and in Sect. 3.2 in Krause et al 2020) that molecular clouds may not be in equilibrium, but rather regions undergoing global hierarchical collapse. The reason is that gravitational collapse has a similar energy signature (α vir ∼ 2) as virial equilibrium (Ballesteros-Paredes et al 2011), since the free-fall velocity is √ 2 times larger than the virial velocity.…”

“…At any level of the hierarchy in the ISM, from GMCs with masses of 10 5 -10 6 M and regions therein with mean densities of n ∼ 10 2 cm −3 , down to dense cores with masses of a few M and densities of n ∼ 10 5 cm −3 , the identified structures typically contain many Jeans masses (see e.g. Krause et al 2020, and references therein). Nonetheless, the simple set of equations provides a timescale estimation for collapse, and thus for star formation within these collapsing regions.…”

“…This transition phase may be long, with mass gain and mass loss being approximately equal, because it may take a few Myr until the star-forming clumps have grown to sufficiently high masses and densities to form massive stars (e.g. Vázquez-Semadeni et al 2017;Krause et al 2020). Eventually, the energy and momentum input from newly formed star-forming regions begins to dominate and the parent cloud is dispersed by stellar feedback (e.g.…”

Star Clusters Near and Far

“…2). We point out the interested reader that a detailed description of the physics of cluster formation and evolution as single entity can be found in Krause et al (2020), another review in this series.…”