Disk accretion onto a magnetized star

TL;DR

This paper provides an exact solution for the interaction between a rotating magnetic star and its accretion disk, revealing how magnetic fields and currents influence disk truncation and stellar spin-up.

Contribution

It offers a novel exact analytical model describing magnetic field compression, current flow, and disk-star interaction in magnetized accretion systems.

Findings

Magnetic field lines are compressed towards the star.

Electric currents flow along specific magnetic surfaces.

Disk rotation is halted at 0.55 of the corotation radius.

Abstract

The problem of interaction of the rotating magnetic field, frozen to a star, with a thin well conducting accretion disk is solved exactly. It is shown that a disk pushes the magnetic field lines towards a star, compressing the stellar dipole magnetic field. At the point of corotation, where the Keplerian rotation frequency coincides with the frequency of the stellar rotation, the loop of the electric current appears. The electric currents flow in the magnetosphere only along two particular magnetic surfaces, which connect the corotation region and the inner edge of a disk with the stellar surface. It is shown that the closed current surface encloses the magnetosphere. Rotation of a disk is stopped at some distance from the stellar surface, which is 0.55 of the corotation radius. Accretion from a disk spins up the stellar rotation. The angular momentum transferred to the star is determined.