Star formation is a clustered process, which naturally leads to the formation of binaries and star clusters. This clustering is most important for massive stars, which most often form in dense clusters and in close binaries. Massive stars are the dominant source of energy in young massive star clusters (YMCs) due to the feedback they return to their environments in the form of winds, radiation, and supernovae. The presence of a close companion affects this feedback by triggering mass transfer and changing the subsequent evolution of massive stars. Stellar dynamics within dense star clusters further affect the binaries by modifying their orbits or disrupting them. In this thesis, we use numerical simulations to investigate the interplay between binary stars and their host clusters during star cluster formation. Using initial conditions typical of the disk of the Milky Way, we find that the clusters undergo rapid morphological changes from subcluster mergers driven by the large-scale gas environment during their formation. Expanding our suite of simulations to include initial conditions typical of starburst galaxies, we find that those mergers lead to a decrease in the binary fraction of low and intermediate mass stars, in agreement with the low binary fractions observed in older massive star clusters. Close massive binaries however remain present even in the densest YMCs. We also present the first implementation of feedback from massive interacting binaries coupled to stellar dynamics. We find that mass transfer in binaries enhances feedback in cluster-forming regions, and that this enhancement cannot be accurately predicted by standalone binary evolution simulations due to the effects of nearby stars and gas on the binaries’ orbits. We conclude that a treatment of stellar dynamics and mass transfer in binaries are essential to understand the formation of massive star clusters in galaxies.

Doctor of Philosophy (PhD)

Thesis