The role of turbulence during the formation of circumbinary discs

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 magnetohydrodynamic simulations with the adaptive mesh refinement code FLASH of binary star formation for separations of 10–20 au. We run a simulation with no initial turbulence (NT), and two with turbulent Mach numbers of M = σv/cs = 0.1 and 0.2 (T1 and T2) for 5000 yr after protostar formation. By the end of the simulations the circumbinary discs in NT and T1, if any, have radii of 20 au with masses 0.02 M, while T2 hosts a circumbinary disc with radius ∼70–80 au and mass ∼0.12 M. 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 outflows of NT carry the most mass, and linear and angular momentum from the system. 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 leads to the restructuring of magnetic fields for efficient outflow launching, too much turbulence may disrupt the ordered magnetic field structure required for magnetocentrifugal launching of jets. We conclude that the role of turbulence in building large circumbinary discs may explain some observed very old (>10 Myr) circumbinary discs. The longer lifetime of circumbinary discs may increase the likelihood of planet formation.