Global 3-D Simulations of Magnetospheric Accretion: II. Hot Spots, Equilibrium Torque, Episodic Wind, and Midplane Outflow

TL;DR

This study uses 3D magnetohydrodynamical simulations to explore how stellar spin affects accretion hot spots, torques, and winds, revealing spin-dependent behaviors and equilibrium states consistent with observations.

Contribution

It provides new insights into the effects of stellar spin on accretion processes, hot spot formation, and wind dynamics through detailed 3D simulations.

Findings

Hot spots are closer to the equator in slow rotators.

Episodic winds have mass-loss rates up to 40% of accretion.

Equilibrium spin occurs at a fastness parameter of about 0.7.

Abstract

Global 3-D magnetohydrodynamical simulations have been conducted to study magnetospheric accretion around stars with various spin rates. For slow rotators, characterized by a fastness parameter $\omega_s\lesssim 0.78$, the disk's inner edge at the magnetospheric truncation radius becomes unstable to the interchange instability, leading to intruding filaments which produce hot spots 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 hot spots are generated only through polar accretion. Low- and mid-energy flux hot spots cover $\lesssim20\%$ and $\lesssim3\%$ of the surface, with faster rotators tending to produce hotter spots. Beyond the truncation radius, angular momentum transfers from the disk surface to the midplane, resulting in surface accretion and midplane outflow. The midplane outflow may transport thermally processed materials (e.g. those in chondrites) to the outer disk. Field inflation generates episodic winds with mass-loss rates $\sim 1\%-40\%$ of the accretion rate, depending on stellar spin. Frequent magnetic reconnections lead to efficient star-disk coupling. We derive the torque exerted by the disk on the star as a function of stellar spin. For fast rotators/propellers, both spin-down torque and disk wind rate increase dramatically with stellar spin. The equilibrium spin state occurs at $\omega_s\sim0.7$, with wind/jet speeds ($\sim$500 km/s) and mass loss rates ($\sim10\%$ accretion rate) aligning with observations. Most results are insensitive to disk thickness. Finally, we present testable predictions for how observables vary with stellar spin.