A class of exact MHD models for astrophysical jets

This paper examines a new class of exact and self-consistent MHD solutions that describe steady and axisymmetric hydromagnetic outflows from the atmos phere of a magnetized and rotating central object with possibly an orbiting accretion disc, The plasma is driven against gravity by a thermal pressure gradient, as well as by magnetic rotator and radiative forces. At the Alfv enic and fast critical points the appropriate criticality conditions are ap plied. The outflow starts almost radially, but after the Alfven transition and before the fast critical surface is encountered, the magnetic pinching force bends the poloidal streamlines into a cylindrical jet-type shape. The terminal speed, Alfven number and cross-sectional area of the jet, as well as its final pressure and density, obtain uniform values at large distance s from the source. The goal of the study is to give an analytical discussio n of the two-dimensional interplay of the thermal pressure gradient, gravit ational, Lorentz and inertial forces in accelerating and collimating an MI- ID flow. A parametric study of the model is also given, as well as a brief sketch of its applicability to a self-consistent modelling of collimated ou tflows from various astrophysical objects. The analysed model succeeds in g iving for the first time an exact MHD solution for jet-type outflows extend ing from the stellar surface to infinity where the outflow can be superfast , in agreement with the Mi-ID causality principle.