MAGNETICALLY DRIVEN JETS AND WINDS

A simple but general theory for the formation and propagation of nonrelativistic jets and winds from magnetized accretion disks is derived from the equations of ideal magnetohydrodynamics (MHD). The theory characterizes a jet by its axial velocity, radius, angular rotation rate, and temperature as functions of axial distance z from the central object. Differential equations for these four variables are derived from the conservation laws of ideal MHD for the flow of mass, momentum, angular momentum, and energy in the jet. The jets are assumed to carry no net current because this is energetically favored. The magnetic field is however essential in that the zz-component of the magnetic stress acts, in opposition to gravity, to "drive" matter through the slow magnetosonic critical point. The centrifugal force has no direct role at this critical point. Close to the disk, the gravitational field of the central object provides weak collimation of the flow. At much larger distances, good collimation can result from the focusing effect of the external medium. Further, the pressure of the external medium can act, in opposition to gravity, to "drive" the flow through the fast magnetosonic critical point. Conservation of mass, angular-momentum, and energy of the disk/jet system significantly constrains the jet solutions. For a representative self-consistent disk/jet solution relevant to a protostellar system, the fraction of the accreted mass expelled in the jets is 0.1, the ratio of the power carried by the jets (kinetic and Poynting fluxes) to the disk luminosity is 0.66, and the ratio of the boundary layer to disk luminosities is less-than-or-similar-to 0.13. The star's rotation rate is found to decrease with time even for rotation rates much less than the breakup rate.