Interaction of Stellar Dipole and Disk Magnetic Fields in Accretion–Ejection Systems

Abstract We study jet formation from a magnetically diffusive, sub-Keplerian accretion disk threaded by different magnetic field configurations. We present two-dimensional, axisymmetric MHD simulations in spherical coordinates employing the PLUTO code. Our results show that the reference simulation, in which the disk is threaded by an initial large-scale poloidal magnetic field, reaches a quasi-steady state and produces a well-collimated jet. Both kinetic and magnetic torques are significant and evolve smoothly over time. The accretion process is established smoothly, with about 26% of the accreting material being delivered into the outflow. Alternatively, applying a purely stellar dipolar magnetic field produces weak and unstable outflows, characterized by inefficient angular momentum transport and strongly suppressed accretion. Additionally, in simulations where the stellar dipolar magnetic field is present alongside the disk’s initial poloidal magnetic field, the disk magnetic field predominantly stabilizes the system. It also facilitates angular momentum transport from the disk and sustains a significant outflow mass flux. Furthermore, we performed additional simulations that explicitly include an inner gap associated with the disk truncation region. These simulations were designed as a robustness test of the treatment of the innermost disk region and confirm that our main conclusions are not sensitive to the presence of this gap. Overall, our findings demonstrate that although the magnitude of stellar dipole plays an important role in the evolution of the accretion–ejection structure, the large-scale poloidal magnetic field threading the disk is essential for launching and sustaining a stable and well-collimated magnetized jet.