The Magnetic Collimation of Bipolar Outflows. I. Adiabatic Simulations

The collimation of an isotropic, nonmagnetic protostellar wind by an ordered magnetic field in the ambient medium is investigated using self-consistent magnetohydrodynamic simulations in two dimensions. The strong protostellar wind inflates a low-density bubble in the ambient medium. The growth of this bubble is confined along the direction of the ambient magnetic field when the gas pressure in the interior drops to the level of the magnetic pressure at the surface. For typical molecular cloud conditions, collimation of the protostellar wind into a bipolar outflow occurs on time scales of 10,000-100,000 yr. When a self-consistent model for a magnetically supported cloud is used as a more realistic initial condition, the wind-blown bubble can 'blowout' along diverging field lines in the direction of the steepest density gradient.