Angular Momentum Transport in Binary Star Formation: The Enhancement of Magnetorotational Instability and Role of Outflows

Abstract The formation of binary stars is highly influenced by magnetic fields, which play a crucial role in transporting angular momentum. We conducted 3D numerical simulations of binary star accretion via a circumbinary disk, taking into account a magnetic field perpendicular to the disk and an infalling envelope. Our simulations reproduce the following phenomena: (1) the spiral arms associated with circumstellar disks; (2) the turbulence in the circumbinary disk, induced by magnetorotational instability (MRI); (3) a fast outflow launched from each circumstellar disk; and (4) a slow outflow from the circumbinary disk. The binary models exhibit a higher α -parameter than the corresponding single-star models, indicating that the binary stars enhance the MRI turbulence. Moreover, an infalling envelope also enhances the turbulence, leading to a high α -parameter. While the spiral arms promote radial flow, causing the transfer of mass and angular momentum within the circumbinary disk, the MRI turbulence and outflows are the main drivers of angular momentum transfer to reduce the specific angular momentum of the system.