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

Methods: We investigate the binary trigger hypothesis in longer-period (>20yr) binaries by carrying out three-dimensional magnetohydrodynamical (MHD) simulations of the formation of low-mass binary stars down to final separations of ~10AU, including the effects of gas turbulence and magnetic fields. We run two simulations with an initial turbulent gas core of one solar mass each and two different initial turbulent Mach numbers, \(\mathcal{M}=\sigma_v/c_s=0.1\) and \(\mathcal{M}=0.2\), for 6500yr after protostar formation.

Results: We observe bursts of accretion at periastron during the early stages when the eccentricity of the binary system is still high. We find that this correlation between bursts of accretion and passing periastron breaks down at later stages, because of the gradual circularisation of the orbits. For eccentricities greater than e=0.2, we observe episodic accretion triggered near periastron. However, we do not find any strong correlation between the strength (the ratio of the burst accretion ratio to the quiescent accretion rate) of episodic accretion and eccentricity. We determine that accretion events are likely triggered by torques between the rotation of the circumstellar disc and the approaching binary stars. We compare our results with observational data of episodic accretion in short-period binaries and find good agreement between our simulations and the observations.

Conclusions: We conclude that episodic accretion is a universal mechanism operating in eccentric young binary-star systems, independent of separation, and should be observable in long-period binaries as well as in short-period binaries, but that the strength will depend on the torques, and hence the separation at periastron.

This animation demonstrates some of the episodic accretion observed at periastron during the early evolution of our simulated binaries. The left shows the total accretion of the binary system. The right panel shows top down density projections of thickness 100AU for the T2 (\(\mathcal{M} =\) 0.2) high resolution (L_ref=12) simulation. The projection is centred on the xy-plane. The thin lines show the magnetic field, and the arrows indicate the velocity field. Crosses show the position of the sink particles. The mass accreted by the sink particles in the simulations is indicated on the bottom left of each panel.

Acknowledgements: The authors thank B. Tofflemire for providing the observational data used in this paper.This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 847523 ‘INTERACTIONS’. R. K. would like to thank the Australian Government and the financial support provided by the Research Training Program Domestic Scholarship. The research leading to these results has received funding from the Independent Research Fund Denmark through grant No. DFF 8021-00350B (RLK). C. F. acknowledges funding provided by the Australian Research Council (Discovery Project DP170100603 and Future Fellowship FT180100495), and the Australia-Germany Joint Research Cooperation Scheme (UA-DAAD). The simulations presented in this work used high performance computing resources provided by the Leibniz Rechenzentrum, the Gauss Centre for Supercomputing (grants pr32lo, pr48pi and GCS Large-scale project 10391), and the Australian National Computational Infrastructure (grant ek9) in the framework of the National Computational Merit Allocation Scheme and the ANU Merit Allocation Scheme. The astrophysics HPC facility at the University of Copenhagen, supported by a research grant (VKR023406) from VILLUM FONDEN, was also used for carrying out part of the simulations, the analysis, and long-term storage of the results. The simulation software \texttt{FLASH} was in part developed by the DOE-supported Flash Center for Computational Science at the University of Chicago. yt (Turk et al, 2011) was used to help visualise and analyse these simulations.