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

Abstract: Most stars form in binaries and the evolution of their discs remains poorly understood. To shed light on this subject, we carry out 3D ideal MHD simulations with the AMR code FLASH of binary star formation for separations of 10 - 20AU. We run a simulation with no initial turbulence (NT), and two with turbulent Mach numbers of \(\mathcal{M} = \sigma_v/c_s =\) 0.1 and 0.2 (T1 and T2) for 5000yr after protostar formation. By the end of the simulations the circumbinary discs in NT and T1, if any, have radii of <20AU with masses <0.02M\(_\odot\), while T2 hosts a circumbinary disc with radius ~70-80AU and mass ~0.12M\(_\odot\). These circumbinary discs formed from the disruption of circumstellar discs and harden the binary orbit. Our simulated binaries launch large single outflows. We find that NT drives the most massive outflows, and also removes large quantities of linear and angular momentum. T2 produces the least efficient outflows concerning mass, momentum and angular momentum (~61 per cent, ~71 per cent, ~68 per cent of the respective quantities in NT). We conclude that while turbulence helps to build circumbinary discs which organise magnetic fields for efficient outflow launching, too much turbulence may also disrupt the ordered magnetic field structure required for magneto-centrifugal launching of jets and outflows. We also see evidence for episodic accretion during the binary star evolution. We conclude that the role of turbulence in building large circumbinary discs may explain some observed very old (>10Myr) circumbinary discs. The longer lifetime of circumbinary discs may increase the likelihood of planet formation.

This animation shows top down density projections of thickness 100AU for the NT (\(\mathcal{M} =\) 0.0, left), T1 (\(\mathcal{M} =\) 0.1, middle) and T2 (\(\mathcal{M} =\) 0.2, right) cases. In all cases 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: R.K. would like to thank the Australian Government and the financial support provided by the Research Training Program Domestic Scholarship. C. F. acknowledges funding provided by the Australian Research Council (Discovery Projects DP150104329 and 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 and the Gauss Centre for Supercomputing (grants pr32lo, pr48pi and GCS Large-scale project 10391), the Partnership for Advanced Computing in Europe (PRACE grant pr89mu), the Australian National Computational Infrastructure (grant ek9), and the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia, in the framework of the National Computational Merit Allocation Scheme and the ANU Allocation Scheme. The simulation software FLASH was in part developed by the DOE-supported Flash Center for Computational Science at the University of Chicago.