The Effects of Field-Aligned Rotation on the Magnetically Channeled Line-Driven Winds

Abstract

There is extensive evidence that the radiatively driven stellar winds of OB-type stars are not the steady, smooth outflows envisioned in classical models, but instead exhibit extensive structure and variability on a range of temporal and spatial scales. We examine the possible role of stellar magnetic fields in forming large-scale wind structure. It is based on numerical magnetohydrodynamic (MHD) simulations of the interaction of a line-driven flow with an assumed stellar dipole field.Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, ‘wind magnetic confinement parameter’, η∗ (= B2eqR2∗/Ṁv∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.We extend these MHD simulations to include field-aligned stellar rotation. The results indicate that a combination of the magnetic confinement parameter and the rotation rate as a fraction of the ‘critical’ rotation now determine the global properties of the wind. For models with strong magnetic confinement, rotation can limit the extent of the last closed magnetic loop, and lead to episodic mass ejections that break through the close loop and are carried outward with a slow, dense, equatorial outflow. Our 2-D numerical simulations indicate that the magnetic fields provide excessive amount of angular momentum to the wind preventing the formation of a Keplerian disk.