On the magnetic acceleration and collimation of astrophysical outflows

The axisymmetric 3-D MHD outflow of cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the outflow’s magnetocentrifugal acceleration and eventual collimation. Gravity and thermal pressure are neglected while a split-monopole is used to describe the initial magnetic field configuration. It is found that the stationary final state depends critically on a single parameter α expressing the ratio of the corotating speed at the Alfvén distance to the initial flow speed along the initial monopole-like magnetic fieldlines. Several angular velocity laws have been used for relativistic and nonrelativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends linearly on α. Significant flow collimation is found in nonrelativistic efficient magnetic rotators corresponding to relatively large values of α> ∼ 1 while very weak collimation occurs in inefficient magnetic rotators with smaller values of α < 1. Part of the flow around the rotation and magnetic axis is cylindrically collimated while the remaining part obtains radial asymptotics. The transverse radius of the jet is inversely proportional to α while the density in the jet grows linearly with α. For α > ∼ 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collimation is found in the supersonic region for parameters typical for radio pulsars. All above results verify the main conclusions of general theoretical studies on the magnetic acceleration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.