On the magnetic acceleration and collimation of astrophysical outflows

The axisymmetric 3D MI-ID outflow of cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the magnetocentrifugal acceleration and eventual collimation of the outflow. 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 a expressing the ratio of the corotating speed at the Alfven distance to the initial flow speed along the initial monopole-like magnetic field lines. Several angular velocity laws have been used for relativistic and non-relativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends Linearly on alpha. Significant flow collimation is found in non-relativistic efficient magnetic rotators corresponding to relatively large values of alpha greater than or similar to 1 while very weak collimation occurs in inefficient magnetic rotators with smaller values of alpha < 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 alpha while the density in the jet grows Linearly with alpha. For alpha greater than or similar to 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 the 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.