Force-Free Models of Magnetically Linked Star-Disk Systems

TL;DR

This paper investigates the evolution of magnetic fields in star-disk systems, highlighting how differential rotation causes field inflation, and explores the physical limits and steady-state conditions of these configurations using simplified and numerical models.

Contribution

It introduces a semianalytic model for magnetic field evolution in star-disk systems, including effects like plasma inertia and reconnection, and extends analysis with a numerical model for rotating disks.

Findings

Magnetic field lines rapidly inflate and open due to differential rotation.

Steady state configurations are unlikely due to physical constraints.

Numerical models confirm the complex evolution of magnetic fields in these systems.

Abstract

Disk accretion onto a magnetized star occurs in a variety of astrophysical contexts, from young stars to X-ray pulsars. The magnetohydrodynamic interaction between the stellar field and the accreting matter can have a strong effect on the disk structure, the transfer of mass and angular momentum between the disk and the star, and the production of bipolar outflows, e.g., plasma jets. We study a key element of this interaction - the time evolution of the magnetic field configuration brought about by the relative rotation between the disk and the star - using simplified, largely semianalytic, models. We first discuss the rapid inflation and opening up of the magnetic field lines in the corona above the accretion disk, which is caused by the differential rotation twisting. Then we consider additional physical effects that tend to limit this expansion, such as the effect of plasma inertia and the possibility of reconnection in the disk's corona, the latter possibly leading to repeated cycles in the evolution. We also derive the condition for the existence of a steady state for a resistive disk and conclude that a steady state configuration is not realistically possible. Finally, we generalize our analysis of the opening of magnetic field lines by using a non-self-similar numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.