MAGNETOHYDRODYNAMIC STELLAR WIND SOLUTIONS IN CURVED MAGNETIC-FIELDS

This paper generalizes the analytic class of magnetohydrodynamic (MHD) solutions introduced by Low and Tsinganos for steady, rotating, axisy mmetric stellar winds embedded in partially open magnetic fields. The collimation of the outflow is achieved by assuming a magnetic configur ation that not only has the property that its field lines are poleward deflected but also includes as particular cases the streamline geomet ries proposed by other authors. The full MHD equations are reduced to a set of second-order, radial, ordinary differential equations by a su itable choice of the angular dependence of the dynamic and thermodynam ic variables. In particular, spherically symmetric Mach-Alfven surface s are assumed. The required heating distribution that can self-consist ently drive the outflow along the prescribed magnetic field is calcula ted; these solutions are found by assuming a polytropic index that dep ends on position. It is shown that provided the wind is properly colli mated unlike the outflows along dipolar magnetic configurations, there now exists a single wind-type solution even in the rotationless case. The behavior of the rotationless solutions is fully explored by varyi ng the wind parameters. The sample of solutions presented illustrates the fact that the asymptotic regime of a rotational atmosphere approac hes that of a rotationless atmosphere because, as is shown, the azimut hal velocity decreases faster than the poloidal velocity in the wind r egion. The topology of the solutions for rotating plasma outflows is d iscussed in detail. Within this framework, the magnetic field line def lection is related to a curvature parameter. Although its interesting values are those close to zero, the curvature parameter takes, in prin ciple, all its allowed values. It is shown that there exists a limitin g curvature in order for the stellar wind to have a physical meaning. The general analysis of theta-dependent Mach-Alfven surfaces will be p resented in a later paper.