The structure of thin accretion discs around magnetised stars

TL;DR

This paper models the steady-state structure of thin accretion discs around magnetized stars, highlighting the dominant role of disc dynamo-generated magnetic fields in star-disc torque and inner edge positioning.

Contribution

It introduces a numerical method to solve the magnetohydrodynamic equations for accretion discs with internal dynamos, revealing the impact of dynamo-generated magnetic fields on disc structure.

Findings

Disc torque is dominated by dynamo contribution.

Inner disc edge varies between R_A and 10 R_A.

Magnetic field strength and direction influence inner edge location.

Abstract

Aims: We determine the steady-state of an axisymmetric thin accretion disc with an internal dynamo around a magnetised star. Methods: Starting from the vertically integrated equations of magnetohydrodynamics we derive a single ordinary differential equation for a thin accretion disc around a massive magnetic dipole and integrate this equation numerically from the outside inwards. Results: Our numerical solution shows that the torque between the star and the accretion disc is dominated by the contribution from the dynamo in the disc. The location of the inner edge of the accretion disc varies between $R_{\rm A}$ and $10R_{\rm A}$ depending mainly on the strength and direction of the magnetic field generated by the dynamo in the disc