Angular Momentum Transport in Binary Star Formation: The Enhancement of Magneto-Rotational 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 three-dimensional 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) spiral arms associated with circumstellar disks, (2) turbulence in the circumbinary disk, induced by magneto-rotational 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 $\alpha$-parameter than the corresponding single star models, indicating that the binary stars enhance MRI turbulence. Moreover, an infalling envelope also enhance the turbulence, leading to a high $\alpha$-parameter. While the spiral arms promotes radial flow, causing transfer of mass and angular momentum within the circumbinary disk, the MRI turbulence and outflows are main drivers of angular momentum transfer to reduce the specific angular momentum of the system.