We implemented a new scheme for primordial binary formation in the star cluster formation code TORCH. We compared simulations with, and without, primordial binaries and followed the dynamical evolution of the binary population in both sets of simulations. We found that both dynamical formation and destruction of binaries take place while the cluster is actively forming new stars. We found that an initial population of binaries is needed at all masses to reproduce observed binary fractions for binaries with mass ratios above the q ≥ 0.1 detection limit. Dynamical interactions in the presence of gas during cluster formation modify the initial distributions towards binaries with smaller primary masses, larger mass ratios, smaller semimajor axes and larger eccentricities. Systems formed dynamically do not have the same properties as the initial systems, and systems formed dynamically in the presence of an initial population of binaries differ from those formed in simulations with single stars only. The results are presented in Cournoyer-Cloutier et al. (2021).

Cumulative distributions of primary masses, mass ratios, semi-major axes and eccentricities for binary systems formed dynamically in simulations without primordial binaries (blue) and in simulations with primordial binaries (yellow), compared to the primordial binaries sampled from observations (black). The distributions are different for all four parameters; the cumulative distributions for dynamical binaries in simulations with primordial binaries always lie between the cumulative distributions for primordial binaries and for dynamically-formed binaries in simulations without primordial binaries.

We performed simulations of star cluster formation to investigate the morphological evolution of embedded star clusters in the earliest stages of their evolution. Our key results are as follows: (1) Cluster mass growth is not monotonic, and clusters can lose up to half of their mass while they are embedded. (2) Cluster morphology is not correlated with cluster mass and changes over ~0.01 Myr time-scales. (3) The morphology of an embedded cluster is not indicative of its long-term evolution but only of its recent history: radius and ellipticity increase sharply when a cluster accretes stars. (4) The dynamical evolution of very young embedded clusters with masses ≲1000 M⊙ is dominated by the overall gravitational potential of the star-forming region rather than by internal dynamical processes such as two- or few-body relaxation. The results are presented in Cournoyer-Cloutier et al. (2023).

3D spatial clustering (left) and ellipsoids enclosing 90% of the mass of the identified bound clusters (right). Member stars for each cluster with at least 100 bound members are shown in a given colour – blue, green, yellow, orange, red, or pink – while unclustered stars are shown in grey.

Massive stars are born in clusters embedded within giant molecular clouds, which they in turn influence via radiative and momentum feedback. Most massive stars are also born in binaries or higher-order systems, with companions close enough to trigger mass loss in excess of what is expected for single stars. Those interactions shape the feedback coming from massive stars, by increasing the amount of ionizing radiation and delaying some of the first core-collapse supernovae. It is therefore crucial to model concurrently those massive interacting binaries and the giant molecular cloud in which they form to reach a more comprehensive understanding of cluster formation. I am currently developping a new model for the implementation of feedback from massive binaries in simulations of cluster formation. This model couples hydrodynamics, stellar dynamics, stellar evolution, and star and binary formation, and innovates by modelling the feedback from both massive single stars and massive binaries. A discussion of our first results and highlighted future outlooks for this novel framework are presented in the slides from this talk at ESO in September 2023.

Energy in different feedback channels (left), and mass loss and remnant masses (right) as a function of time for a binary with stars of masses 80 and 40 M⊙, with semi-major axis of 0.5 AU and eccentricity of 0.5. The solid lines correspond to the output from binary evolution while the dotted lines correspond to the output if the two stars are involved using single-star models.