On the magnetic acceleration and collimation of astrophysical outflows

Abstract

ABSTRA C T The axisymmetric 3D MHD 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 a. Significant flow collimation is found in non-relativistic efficient magnetic rotators corresponding to relatively large values of a * 1 while very weak collimation occurs in inefficient magnetic rotators with smaller values of a < 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 a while the density in the jet grows linearly with a .F ora * 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collima- tion 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 accel- eration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.