Dynamical Simulations of Magnetically Channeled Line‐driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow

We present numerical magnetohydrodynamic (MHD) simulations of the effect ofstellar dipole magnetic fields on line-driven wind outflows from hot, luminousstars. Unlike previous fixed-field analyses, the simulations here take fullaccount of the dynamical competition between field and flow, and thus apply toa full range of magnetic field strength, and within both closed and openmagnetic topologies. A key result is that the overall degree to which the windis influenced by the field depends largely on a single, dimensionless, `windmagnetic confinement parameter', $\eta_{\ast}$ ($ = B_{eq}^2 R_\ast^2/{\dot M}v_\infty$), which characterizes the ratio between magnetic field energy densityand kinetic energy density of the wind. For weak confinement $\eta_{\ast} \le1$, the field is fully opened by the wind outflow, but nonetheless forconfinements as small as $\eta_{\ast}=1/10$ can have a significantback-influence in enhancing the density and reducing the flow speed near themagnetic equator. For stronger confinement $\eta_{\ast} > 1$, the magneticfield remains closed over a limited range of latitude and height about theequatorial surface, but eventually is opened into a nearly radial configurationat large radii.