2-DIMENSIONAL MHD MODELS FOR STELLAR WINDS

We present a new class of exact solutions of the steady axisymmetric M HD equations relevant for stellar winds. The geometry of the streamlin es and fieldlines is helicoidal. All quantities depend both on distanc e to the central object and on latitude. A technique based on a nonlin ear separation of variables yields the most general solution which dep ends on latitude vis three anisotropy parameters. These are related to typical values of the different quantities st the base of the outflow . We are thus able to model a wide range of winds from almost spherica lly symmetric to highly anisotropic ones. Topologically, there are two critical points present in the solution for the radial dependence of the outflow speed. One appears at the familiar star-point Alfvenic sin gularity while the other is at the X-type singularity where the radial how speed equals the fast MHD wave speed in the radial direction. A u nique solution corresponding to zero pressure at infinity must pass th rough both these two points. For the wind to be able to accelerate to large distances, the density at the equator must exceed the density at the pole. In these circumstances, the polar speed is always greater t han the equatorial one. This trend is confirmed by the Ulysses data on the heliolatitudinal dependence of the solar wind as well by interpla netary scintillation data.