Global 3D simulations of magnetospheric accretion – II. Hotspots, equilibrium torque, episodic wind, and mid-plane outflow
Zhaohuan Zhu
Abstract
ABSTRACT Global 3D magnetohydrodynamical simulations have been conducted to study magnetospheric accretion around stars with various spin rates. For slow rotators, characterized by a fastness parameter $\omega _{\mathrm s}\lesssim 0.78$, the disc’s inner edge at the magnetospheric truncation radius becomes unstable to the interchange instability, leading to intruding filaments which produce hotspots closer to the stellar equator. Depending on spin rate, slow rotators can be in ‘chaotic’ or ‘ordered’ unstable regimes. For fast rotators, the interchange instability is suppressed by the super-Keplerian rotation beyond the corotation radius, and hotspots are generated only through polar accretion. Low- and mid-energy flux hotspots cover $\lesssim 20~{{\rm per\,cent}}$ and $\lesssim 3~{{\rm per\,cent}}$ of the surface, with faster rotators tending to produce hotter spots. Beyond the truncation radius, angular momentum transfers from the disc surface to the mid-plane, resulting in surface accretion and mid-plane outflow. The mid-plane outflow may transport thermally processed materials (e.g. those in chondrites) to the outer disc. Field inflation generates episodic winds with mass-loss rates $\sim$1–40 per cent of the accretion rate, depending on stellar spin. Frequent magnetic reconnections lead to efficient star–disc coupling. We derive the torque exerted by the disc on the star as a function of stellar spin. For fast rotators/propellers, both spin-down torque and disc wind rate increase dramatically with stellar spin. The equilibrium spin state occurs at $\omega _{\mathrm s}\sim 0.7$, with wind/jet speeds ($\sim$500 km s−1) and mass-loss rates ($\sim 10~{{\rm per\,cent}}$ accretion rate) aligning with observations. Most results are insensitive to disc thickness. Finally, we present testable predictions for how observables vary with stellar spin.