Theory of jets from young stars

Simple equations are derived for the long-distance propagation of magnetohydrodynamic (MHD) jets. Solutions of these equations are fitted to two observed jets providing estimates of the fast magnetosonic speeds (V(f)) and the distances of the fast magnetosonic points. The relation of the jet properties at large distances to a complete family of MHD jet solutions is discussed, and it is shown that there is one key dimensionless parameter, B. The dependences of the fast magnetosonic speed and of the fluxes of mass, energy, momentum, and angular momentum of the jet on B are discussed. For B larger than a critical value (about 0.45), the central star spins down, while for smaller values it spins up. For increasing from the critical value, V(f) increases while the mass and momentum fluxes of the jet decrease.