Unstable Disk Accretion to Magnetized Stars: First Global 3D MHD Simulations

TL;DR

This paper presents the first 3D MHD simulations of disk accretion onto magnetized stars, revealing how accretion regimes depend on rate and magnetic tilt, with implications for observed stellar light-curves.

Contribution

It introduces novel 3D simulations showing the transition between stable and unstable accretion regimes based on accretion rate and magnetic tilt.

Findings

Unstable accretion involves matter penetrating via elongated tongues.

Stable accretion occurs through ordered funnel streams.

Stars switch between regimes, affecting light-curve patterns.

Abstract

We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small angle Theta. Simulations show that, depending on the accretion rate, a star may be in the stable or unstable regime of accretion. In the unstable regime, matter penetrates deep into the magnetosphere through several elongated "tongues" which deposit matter at random places on the surface of the star, leading to stochastic light-curves. In the stable regime, matter accretes in ordered funnel streams and the light-curves are almost periodic. A star may switch between these two regimes depending on the accretion rate and may thus show alternate episodes of ordered pulsations and stochastic light-curves. In the intermediate regime, both stochastic and ordered pulsations are observed. For Theta > 30 degrees, the instability is suppressed and stable accretion through funnel streams dominates.