The Contribution of Core-fragmentation on protostellar multiplicity

Context. Observations of young multiple star systems find a bimodal distribution in companion frequency and separation. The origin of these peaks have often been attributed to binary formation via core and disc-fragmentation. However, theory and simulations suggests that young stellar systems that form via core-fragmentation undergo significant orbital evolution. Aims. Using simulations of star formation in giant molecular clouds we investigate the influence of environment on multiple star formation pathways and the contribution of core-fragmentation is on the formation of close (< 100 AU) binaries. Methods. Simulations are run with the adaptive mesh refinement code RAMSES with sufficient resolution to resolve core-fragmentation beyond 400 AU and dynamical evolution down to 16 AU, but without the possibility of resolving disc-fragmentation. The evolution of the resulting stellar systems is followed over millions of years. Results. We find that star formation in lower gas density environments is more clustered, but despite this, the fractions of systems that form via dynamical capture and core-fragmentation are broadly consistent at ∼40% and ∼60% respectively. In all gas density environments, we find the typical scale at which systems form via core-fragmentation is 103−3.5 AU. After formation, we find that systems that form via core-fragmentation have slightly lower inspiral rates (∼ 10−1.75 AU/yr measured over first 10000 yr) compared to dynamical capture (∼ 10−1.25 AU/yr). We then compared the simulation with conditions most similar to the Perseus star forming region to determine whether the bimodal distribution observed by Tobin et al. (2016b) can be replicated. We find that it can be replicated, but it is sensitive to the evolutionary state of the simulation. Conclusions. Our results indicate that a significant number of binary star systems with separations < 100 AU can be produced via non-disc-fragmentation pathways due to efficient inspiral, suggesting disc-fragmentation is not the dominant formation pathway for low-mass close binaries in nature.