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Many of the previous studies hypothesize that aprotostar and its protoplanetary disk are fedaxisymmetrically from their envelope. However, recent observations suggest that disks canencounter molecular cloudlets and receive additional mass and angular momentum duringtheir evolution.Hydrodynamic simulations have been performed to study such a cloudletcapture 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 fieldsgenerally extract the angular momentum of accreting materials. However, we found thatmagnetic tension force can not only decelerate but also accelerate the rotational motion of thecloudlet 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).

Young stars such as protostars and pre-main-sequence stars evolve via the interaction with the surrounding accretion disks. It is believed that stellar and disk magnetic fields play important roles in shaping the accretion structure and exchanging the angular momentum between the stars and the disks. However, because of the complexity of gas dynamics around the stars, the star-disk interaction remains poorly understood, which makes the construction of the stellar evolution models difficult. To reveal the interaction processes, we have been performing 3D magnetohydrodynamic simulations of accretion onto a young star with different stellar magnetic fields. In the case of a weakly magnetized, magnetosphere-free star, we found that failed disk wind becomes supersonic, high-latitude accretion flows onto the star (Takasao et al. 2018). This result may explain the reason why Herbig Ae/Be stars show fast accretion. In a different model with stronger disk fields, we showed that the star can produce recurrent explosions via magnetic reconnection (Takasao et al. 2019). We consider that the mechanism is relevant to protostellar flares in class-0/I protostars. In addition to the above two models, we have been investigating the magnetospheric accretion which is very relevant to classical T-Tauri stars. In this talk, we will introduce our 3D modeling and discuss how the star-disk interaction changes depending on the stellar and disk field strengths., {"references":["Kulkarni, A. K. and Romanova, M. M. (2008). Accretion to magnetized stars through the Rayleigh-Taylor instability: global 3D simulations","Takasao, Shinsuke et al. (2018). A Three-dimensional Simulation of a Magnetized Accretion Disk: Fast Funnel Accretion onto a Weakly Magnetized Star","Takasao, Shinsuke et al. (2019). Giant Protostellar Flares: Accretion-driven Accumulation and Reconnection-driven Ejection of Magnetic Flux in Protostars","Cauley, P. Wilson; Johns-Krull, Christopher M. (2014). Diagnosing Mass Flows around Herbig Ae/Be Stars Using the He I λ10830 Line","Stone, James M. et al. (2020). The Athena++ Adaptive Mesh Refinement Framework: Design and Magnetohydrodynamic Solvers"]}

Generally it has been assumed that the presence of a fast (close to the escape velocity) accretion is an indication of the magnetospheric accretion. However, observations indicate that fast accretion also occurs even in a weakly magnetized stars like Herbig Ae stars, which poses a question about the picture of accretion we have developed. We performed 3D MHD simulations by using the Athena++ code, and analyzed the accretion from an MRI (Magneto-Rotational Instability)-active disk onto a weakly magnetized star. As a result, we found that fast accretion to a high-latitude, which is similar to the magnetospheric accretion, is possible even without the stellar magnetosphere. Our results suggest a possibility that stars without the magnetosphere can show a violent accretion behavior associated with X-ray activities. We will discuss the physics of the accretion on the basis of our simulations.