A Global 3-D Simulation of Magnetospheric Accretion: I. Magnetically Disrupted Disks and Surface Accretion

TL;DR

This paper presents a comprehensive 3-D MHD simulation of magnetospheric accretion onto a non-rotating star, revealing complex filamentary structures, magnetic bubbles, and surface accretion regions, with implications for star-disk interactions and planet migration.

Contribution

It introduces a detailed 3-D simulation capturing the intricate structures and mechanisms of magnetospheric accretion, including filament formation, magnetic bubbles, and surface accretion, challenging traditional models.

Findings

Filaments penetrate deep into the magnetosphere and impact the star at ~30° from the poles.

Large-scale magnetic bubbles orbit and cause asymmetric mass ejection.

Accretion rate remains steady with 23% variability.

Abstract

We present a 3-D ideal MHD simulation of magnetospheric accretion onto a non-rotating star. The accretion process unfolds with intricate 3-D structures driven by various mechanisms. First, the disc develops filaments at the magnetospheric truncation radius ($R_T$) due to magnetic interchange instability. These filaments penetrate deep into the magnetosphere, form multiple accretion columns, and eventually impact the star at $\sim$30$^o$ from the poles at nearly the free-fall speed. Over 50% (90%) of accretion occurs on just 5% (20%) of the stellar surface. Second, the disc region outside $R_T$ develops large-scale magnetically dominated bubbles, again due to magnetic interchange instability. These bubbles orbit at a sub-Keplerian speed, persisting for a few orbits while leading to asymmetric mass ejection. The disc outflow is overall weak because of mostly closed field lines. Third, magnetically-supported surface accretion regions appear above the disc, resembling a magnetized disc threaded by net vertical fields, a departure from traditional magnetospheric accretion models. Stellar fields are efficiently transported into the disc region due to above instabilities, contrasting with the ``X-wind'' model. The accretion rate onto the star remains relatively steady with a 23% standard deviation. The periodogram reveals variability occurring at around 0.2 times the Keplerian frequency at $R_T$, linked to the large-scale magnetic bubbles. The ratio of the spin-up torque to $\dot{M}(GM_*R_T)^{1/2}$ is around 0.8. Finally, after scaling the simulation, we investigate planet migration in the inner protoplanetary disc. The disc driven migration is slow in the MHD turbulent disc beyond $R_T$, while aerodynamic drag plays a significant role in migration within $R_T$.