A quasi-one-dimensional model for narrow astrophysical jets

A quasi-one-dimensional model for narrow jets is developed in which confinement of the flow, assumed to rotate about the jet axis, is caused by the tension of the frozen-in magnetic field. The model is obtained by projecting the system of time-independent MHD equations onto the axis of the jet. The resulting three-parameter system consists of three first-order differential equations for the radius and velocity of the jet, together with algebraic relations for the remaining quantities of interest. The analytic structure of the equations is studied and the system is solved for the special case of a constant jet opening angle, using parameters appropriate to jets emanating from black holes, neutron stars, and protostars. For the black hole case, acceleration to v(max) of roughly 0.3c occurs over a distance of a few 100 AU from the source. For the neutron star case, the jet accelerates to v(max) of roughly 0.96c, at a distance of 1.6 x 10 to the 10th cm. 36 refs.