Magnetic collimation in planetary nebulae

Recent studies have focused on the role of initially weak toroidal magnetic fields embedded in a stellar wind as the agent for collimation in planetar y nebulae. In these models, the wind is assumed to be permeated by a helica l magnetic field in which the poloidal component falls off faster than the toroidal component. The collimation occurs only after the wind is shocked a t large distances from the stellar source. In this paper we reexamine assum ptions built into this "magnetized wind-blown bubble" (MWBB) model. We show that a self-consistent study of the model leads to a large parameter regim e in which the wind is self-collimated before the shock wave is encountered . We also explore the relation between winds in the MWBB. model and those t hat are produced via magnetocentrifugal processes. We conclude that a more detailed examination of the role of self-collimation is needed in the conte xt of planetary nebula studies.