MHD model of nonaxisymmetric accretion from an envelope to a protoplanetary disk

Many of the previous studies hypothesize that a protostar and its protoplanetary disk are fed axisymmetrically from their envelope. However, recent observations suggest that disks can encounter molecular cloudlets and receive additional mass and angular momentum during their evolution. Hydrodynamic simulations have been performed to study such a cloudlet capture event on a thousand au scale (Dullemond et al. 2019; Küffmeier et al. 2020). However, the interaction on a disk (∼100 au) scale has not been investigated numerically. As the capture event will be affected by magnetic force, we performed 3D MHD simulations of the interaction between a disk and a magnetized cloudlet. It is commonly expected that magnetic fields generally extract the angular momentum of accreting materials. However, we found that magnetic tension force can not only decelerate but also accelerate the rotational motion of the cloudlet around the star, depending on the ratio of the disk thickness and the cloudlet size. Our results indicate that a part of the cloudlet material captured by the disk is accelerated if the vertical size of the cloudlet is larger than the disk thickness. We also found that a fraction of the cloudlet material becomes gravitationally unbound due to the magnetic acceleration, which may be relevant to the spiral arm around RU Lup (Huang et al. 2020).