A prescription and fast code for the long-term evolution of star clusters - III. Unequal masses and stellar evolution

TL;DR

This paper introduces an enhanced version of the EMACSS code that models the long-term evolution of star clusters with unequal stellar masses and stellar evolution effects, validated against N-body simulations.

Contribution

The new EMACSS version incorporates stellar evolution and unequal masses, providing accurate, fast predictions of cluster evolution in various environments.

Findings

EMACSS accurately reproduces N-body simulation results.

Stellar evolution causes mean stellar mass to decrease and cluster radius to expand.

Balanced evolution leads to faster star escape and mass segregation.

Abstract

We present a new version of the fast star cluster evolution code Evolve Me A Cluster of StarS (EMACSS). While previous versions of EMACSS reproduced clusters of single-mass stars, this version models clusters with an evolving stellar content. Stellar evolution dominates early evolution, and leads to: (1) reduction of the mean mass of stars due to the mass loss of high-mass stars; (2) expansion of the half-mass radius; (3) for (nearly) Roche Volume filling clusters, the induced escape of stars. Once sufficient relaxation has occurred (~ 10 relaxation times-scales), clusters reach a second, 'balanced' state whereby the core releases energy as required by the cluster as a whole. In this state: (1) stars escape due to tidal effects faster than before balanced evolution; (2) the half-mass radius expands or contracts depending on the Roche volume filling factor; and (3) the mean mass of stars increases due to the preferential ejection of low-mass stars. We compare the EMACSS results of several cluster properties against N-body simulations of clusters spanning a range of initial number of stars, mass, half-mass radius, and tidal environments, and show that our prescription accurately predicts cluster evolution for this database. Finally, we consider applications for EMACSS, such as studies of galactic globular cluster populations in cosmological simulations.