Stellar winds, dead zones, and coronal mass ejections

Axisymmetric stellar wind solutions are presented that were obtained by numerically solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are critically analyzed using the knowledge of the flux functions. These flux functions enter in the general variational principle governing all axisymmetric stationary ideal MI-ID equilibria. The magnetized wind solutions for (differentially) rotating stars contain both a wind and a dead zone. We illustrate the influence of the magnetic field topology on the wind acceleration pattern by varying the coronal field strength and the extent of the dead zone. This is evident from the resulting variations in the location and appearance of the critical curves for which the wind speed equals the slow, Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic acceleration to super-Alfvenic velocities as the polar, open field lines are forced to fan out rapidly with radial distance. A higher field strength moves the Alfven transition outward. In the ecliptic, the wind outflow is clearly modulated by the extent of the dead zone. The combined effect of a fast stellar rotation and an equatorial dead zone in a bipolar field configuration can lead to efficient thermocentrifugal equatorial winds. Such winds show both a strong poleward collimation and some equatorward streamline bending due to significant toroidal field pressure at midlatitudes. We discuss how coronal mass ejections are then simulated on top of the transonic outflows.