Stellar Winds, Dead Zones, and Coronal Mass Ejections

Axisymmetric stellar wind solutions are presented, obtained by numericallysolving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions arecritically analysed using the knowledge of the flux functions. These fluxfunctions enter in the general variational principle governing all axisymmetricstationary ideal MHD equilibria. The magnetized wind solutions for(differentially) rotating stars contain both a `wind' and a `dead' zone. Weillustrate the influence of the magnetic field topology on the windacceleration pattern, by varying the coronal field strength and the extent ofthe dead zone. This is evident from the resulting variations in the locationand appearance of the critical curves where the wind speed equals the slow,Alfven, and fast speed. Larger dead zones cause effective, fairly isotropicacceleration to super-Alfvenic velocities as the polar, open field lines areforced to fan out rapidly with radial distance. A higher field strength movesthe Alfven transition outwards. In the ecliptic, the wind outflow is clearlymodulated by the extent of the dead zone. The combined effect of a fast stellarrotation and an equatorial `dead' zone in a bipolar field configuration canlead to efficient thermo-centrifugal equatorial winds. Such winds show both astrong poleward collimation and some equatorward streamline bending due tosignificant toroidal field pressure at mid-latitudes. We discuss how coronalmass ejections are then simulated on top of the transonic outflows.