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 a spects of the physics of hot and magnetized outflows which originate in the near environment of a central gravitating body. By analyzing the asymptoti cal behaviour of the outflows it is found that they attain a variety of sha pes such as conical, paraboloidal or cylindrical. However, non cylindrical asymptotics can be achieved only when the magnetic pinching is negligible a nd the outflow is overpressured on its symmetry axis. In cylindrical jet-ty pe asymptotics, the outflowing plasma reaches an equilibrium wherein it is confined by magnetic forces or gas pressure gradients, while it is supporte d by centrifugal forces or gas pressure gradients. In which of the two regi mes (with thermal or magnetic confinement) a jet can be found depends on th e efficiency of the central magnetic rotator. The radius and terminal speed of the jet are analytically given in terms of the variation across the pol oidal streamlines of the total energy. Large radius of the jet and efficien t 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 wavel engths, as also found by other analytical models. Scenarios for the evoluti on of outflows into winds and jets in the different confinement regimes are shortly outlined.