The effects of magnetic fields and field-aligned rotation on line-driven hot-star winds

There is extensive evidence that the line-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. This thesis examines the possible role of stellar magnetic fields in forming large-scale wind structure based on numerical magnetohydrodynamic (MHD) simulations of the interaction of a line-driven flow with an assumed stellar dipole field.; The first two chapters provide a historical overview and a background summary of the dynamics of line-driven winds. Chapter 3 presents initial MHD simulations of the effect of a dipole field on isothermal models of such line-driven outflows. Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between field and flow. 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’, $h*$ (= ), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.; Chapter 4 analyzes the properties of these simulations, with focus on the effect of magnetic field tilt on the mass flux and rapid flow-tube divergence on the terminal flow speed. The results show that previous expectations of factor 2–3 enhancement were a consequence of assuming a point-star approximation for the wind driving, and that in finite-disk-corrected models one obtains only 20–30% speed increase.; Chapter 5 extends our 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. In contrast to these idealized isothermal models, wherein any hot gas is assumed to radiate away excess energy instantaneously, Chapter 6 carries out MHD simulations of the other extreme limit of adiabatic outflows, for which no energy is lost at all. The results show that adiabatic models with magnetic confinement $h*$ < 1 are very similar to their isothermal counterparts, but those with $h*$ ≥ 1 are dramatically different from the isothermal case, with much greater level of equatorial confinement.