Magnetic shaping of Planetary Nebulae

Summary

Abstract

As in the case of the solar wind, the magnetic field in the wind from a magnetized, rotating star becomes increasingly toroidal with distance from the star. The strength of the magnetic field can he characterized by σ, the ratio of toroidal magnetic energy density to kinetic energy density in the equatorial plane of the wind. A fast wind shocks against the external medium and creates a bubble whose volume is dominated by shocked gas. The toroidal magnetic field increases in the shocked bubble and can dominate the thermal pressure. Because of the low velocities in the bubble, hydrostatic equilibrium is a good approximation and allows the calculation of the thermal and magnetic pressure in the bubble, as in the model of Begelman & Li (1992) for the Crab Nebula. The pressure is asymmetric because magnetic tension constrains the flow in the equatorial direction and there are no magnetic effects in the polar direction. The total pressure drives a shell into the surrounding medium, which can be treated in the axisymmetric “thin shell” approximation. If the fast wind is running into a slow wind from a previous evolutionary phase, the interaction shell tends toward motion at constant velocity and the shell structure varies only with polar angle. The structure, which is axisymmetric and extended in the polar direction, depends on 2 parameters: σv_w/w_o, where v_u, is the wind velocity and w_o is the shell velocity in the polar direction, and λ = v_a/w_o, where v_a is the velocity of the slow wind. […]