Outflows from magnetic rotators

The asymptotic structure of outflows from rotating magnetized objects confined by a uniform external pressure is calculated. The flow is assumed to be perfect MHD, polytropic, axisymmetric and stationary. The well known associated first integrals together with the confining external pressure determine the asymptotic structure. The integrals are provided by solving the flow physics for the base within the framework of the model developed in Paper I (Lery et al. 1998), which assumes conical geometry below the fast mode surface, transfield equilibrium being demanded only at the Alfvén surface, where it takes the form of the Alfvén regularity condition. This model can accept any set of boundary conditions. Its solutions are parameterized by the distribution with respect to flux of rotation rate Ω(a), specific entropy Q(a) at the base of the jet, and by the mass to magnetic flux ratio on the polar axis α 0. Having obtained the first integrals in the inner region, the external confining pressure enters as the only parameter of the asymptotic problem. We have taken it as independent of the distance to the wind source object, which results in an asymptotically cylindrical geometry. It is found that slow (i.e. with small rotation parameter ω) rigid rotators give rise to diffuse electric current distribution in the asymptotic region and are dominated by gas pressure. On the other hand, fast rigid rotators have a core-envelope structure in which a current carrying core is surrounded by an essentially current free region where the azimuthal magnetic field dominates. When the confining external pressure is made to decrease towards zero, the total asymptotic poloidal current carried in the wind zone decreases steadily, but does not seem to asymptote to a finite value for strictly zero outer pressure. A sizeable finite current remains present however for fast rotators even at exceedingly small, but still finite, pressure.