Where do binary stars come from?
Most observed stars are part of a multiple star system, i.e., in binary, triple or quadrouple systems. It is generally understood that these systems form either during the star-forming phase of the parent cloud, where the dominant formation channels are the fragmentation of a protostellar core or disk (which ensures that the stars form next to each other), or through dynamical evolution during the dissolution of the cluster (e.g., a massive star capturing a nearby lower mass star). Despite deacdes of research, the formation of such systems and the role of environment and various physical processes is still poorly understood.
In this work we study the multiplicity properties of stars in a suite of STARFORGE simulations that include stellar feedback with varied initial surface density, magnetic fields, level of turbulence, metallicity, interstellar radiation field, simulation geometry and turbulent driving. In our fiducial cloud the raw simulation data reproduces the observed multiplicity fractions for Solar-type and higher mass stars, similar to previous works. However, after correcting for observational incompleteness the simulation under-predicts these values. The discrepancy is likely due to the lack of disk fragmentation, as the simulation only resolves multiples that form either through capture or core fragmentation. The raw mass distribution of companions is consistent with randomly drawing from the initial mass function for the companions of stars above a Solar mass, however, accounting for observational incompleteness produces a flatter distribution similar to observations. We show that stellar multiplicity changes as the cloud evolves and anti-correlates with stellar density. This relationship also explains most multiplicity variations between runs, i.e., variations in the initial conditions that increase stellar density (increased surface density, reduced turbulence) decrease multiplicity. While other parameters, such as metallicity, interstellar radiation, and geometry significantly affect the star formation history or the IMF, varying them produces no clear trend in stellar multiplicity properties.