Dissertation: Dynamics and Feedback of Massive Binaries
by Dr Claude Cournoyer-Cloutier
Thesis defended on July 30, 2025
Department of Physics & Astronomy
McMaster University
Thesis advisor: Dr. Alison Sills
Abstract:
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 thesubsequent 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 ofnearby 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.
I found the discussion about how massive binary stars influence the formation of young star clusters particularly interesting. The explanation of how mass transfer between close binary stars can enhance feedback through winds, radiation and other processes made the research easier to appreciate. I also liked the point that stellar dynamics and nearby gas can change binary orbits, meaning that simulations need to consider more than just individual stars. When reading about complex research projects, even topics like dissertation help australia highlight how important it is to understand the methodology behind academic work. It would be interesting to see how these simulation findings compare with observations of actual star clusters.