Winds from luminous late-type stars. I. The effects of nonlinear Alfven waves.

We present the results of magnetohydrodynamic (MHD) modeling of winds from luminous late-type stars using a 2.5-dimensional, nonlinear MHD computer code. We assume that the wind is generated within an initially hydrostatic atmosphere and is driven by torsional Alfven waves generated at the stellar surface. Two cases of atmospheric topology are considered: case I has longitudinally uniform density distribution and isotropic radial magnetic field over the stellar surface, and case II has an isotropic, radial magnetic field with a transverse density gradient, which we refer to as an atmospheric hole. We use the same set of boundary conditions for both models. The calculations are designed to model a cool luminous star, for which we assume an initial hydrostatic pressure scale height of 0.072 R-*, an Alfven wave speed of 92 km s(-1) at the surface, and a wave period of 76 days, which roughly corresponds with the convective turnover time. For case I the calculations produce a wind with terminal velocity of similar to 22 km s(-1) and a mass loss rate comparable to the expected value of 10(-6) M-circle dot yr(-1). For case II we predict a two-component wind: a fast (25 km s(-1)) and relatively dense wind outside of the atmospheric hole and a slow (15 km s(-1)), rarefied wind inside of the hole.