Effect of a magnetic field on massive star winds I: mass-loss and velocity for a dipole field

TL;DR

This paper develops a new formalism to analyze how magnetic dipole fields influence stellar wind mass-loss rates and velocities, revealing effects of optical-thin corrections and rotation, and challenging previous models of magnetosphere velocity structures.

Contribution

It introduces the ARFHD formalism for arbitrary magnetic topologies and provides new scaling laws for mass-flux affected by magnetic fields and rotation.

Findings

Optically-thin correction decreases mass-loss and wind acceleration.

Rotation enhances mass-loss and wind velocity.

Magnetosphere velocity structure differs from the beta-velocity law.

Abstract

We generalize the Rigid-Field Hydrodynamic equations to accommodate arbitrary magnetic field topologies, resulting in a new Arbitrary Rigid-Field hydrodynamic (ARFHD) formalism. We undertake a critical point calculation of the steady-state ARFHD equations with a CAK-type radiative acceleration and determine the effects of a dipole magnetic field on the usual CAK mass-loss rate and velocity structure. Enforcing the proper optically-thin limit for the radiative line-acceleration is found to decrease both the mass-loss and wind acceleration, while rotation boosts both properties. We define optically-thin-correction and rotation parameters to quantify these effects on the global mass-loss rate and develop scaling laws for the surface mass-flux as a function of surface colatitude. These scaling laws are found to agree with previous laws derived from magnetohydrodynamic simulations of magnetospheres. The dipole magnetosphere velocity structure is found to differ from a global beta-velocity law, which contradicts a central assumption of the previously-developed XADM model of X-ray emission from magnetospheres.