I am Rajika Kuruwita, an Independant Postdoc Fellow
at the Heidelberg Institute for Theoretical Studies

Abstract: Context: Observations of young multiple star systems find a bimodal distribution in companion frequency and separation. The origin of these peaks has often been attributed to binary formation via core and disc fragmentation. However, theory and simulations suggest that young stellar systems that form via core fragmentation undergo significant orbital evolution. Aims: We investigate the influence of the environment on the formation and orbital evolution of multiple star systems, and how core fragmentation contributes to the formation of close (20 − 100AU) binaries. We use multiple simulations of star formation in giant molecular clouds and compare them to the multiplicity statistics of the Perseus star-forming region. Methods: Simulations were run with the adaptive mesh refinement code RAMSES with sufficient resolution to resolve core fragmentation beyond 400AU and dynamical evolution down to 16.6AU, but without the possibility of resolving disc fragmentation. The evolution of the resulting stellar systems was followed over millions of years. Results: We find that star formation in lower gas density environments is more clustered; however, 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 that 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 (∼ 10−1.68 AU/yr measured over the first 10000 yr) compared to dynamical capture (∼ 10−1.32 AU/yr). We then compared the simulation with the conditions most similar to the Perseus star-forming region to determine whether the observed bimodal distribution 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 low-mass close binaries with separations from 20 − 100AU can be produced via core fragmentation or dynamical capture due to efficient inspiral, without the need for a further contribution from disc fragmentation.

Shows the median companion frequency (CF) vs separation averaged over an SFE ±0.1% window, for the M_gas = 3750M_\odot simulation. The solid histograms show the resulting CF vs. Separation histogram, the blue and orange show the results with an upper limit of 55 and 120 L_\odot respectively. The black dashed histogram shows the observed distribution found by Tobin et al. (2022) for all class 0/I objects, and the solid black line is the histogram derived from pairs with boundness likelihood > 0.68. The error bars are calculated using the binomial statistics described by equation 3 in Tobin et al. (2022).

Acknowledgements: We thank the anonymous referee for their insightful comments and suggestions. The research leading to these results has received funding from the Independent Research Fund Denmark through grant No. DFF 8021-00350B (TH, RLK). This project has received funding from the European Unions Horizon 2020 research and innovation Program under the Marie Sklodowska-Curie grant agreement No. 847523 INTERACTIONS. The astrophysics HPC facility at the University of Copenhagen, supported by research grants from the Carlsberg, Novo, and Villum foundations, was used for carrying out the simulations and analysis, as well as long-term storage of the results. RLK also acknowledges funding from the Klaus Tschira Foundation. yt (Turk et al. 2011) was used to help visualise and analyse these simulations.