Nonradial and nonpolytropic astrophysical outflows IV. Magnetic or thermal collimation of winds into jets?

An axisymmetric MHD model is examined analytically to illustrate some key aspects of the physics of hot and magnetized outflows which originate in the near environment of a central gravitating body. By analyzing the asymptotical behaviour of the outflows it is found that they attain a variety of shapes such as conical, paraboloidal or cylindrical. However, non cylindrical asymptotics can be achieved only when the magnetic pinching is negligible and the outflow is overpressured on its symmetry axis. In cylindrical jet-type asymptotics, the outflowing plasma reaches an equilibrium wherein it is confined by magnetic forces or gas pressure gradients, while it is supported by centrifugal forces or gas pressure gradients. In which of the two regimes (with thermal or magnetic confinement) a jet can be found depends on the efficiency of the central magnetic rotator. The radius and terminal speed of the jet are analytically given in terms of the variation across the poloidal streamlines of the total energy. Large radius of the jet and efficient acceleration are best obtained when the external confinement is provided with comparable contributions by magnetic pinching and thermal pressure. In most cases, collimated streamlines undergo oscillations with various wavelengths, as also found by other analytical models. Scenarios for the evolution of outflows into winds and jets in the different confinement regimes are shortly outlined.