Driving Outflows from Young Stars through the Effects of Internal Disk Fields

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Abstract
We examine the evolution of magnetized, differentially rotating buoyant elements ejected through the surface of an accretion disk into an unmagnetized ambient medium, using axisymmetric magnetohydrodynamic calculations. The evolution occurs in three distinct stages. First, angular momentum transfer along radial magnetic field lines allows part of each element to plunge toward the rotation axis. Next, the vertical gradient in total pressure accelerates some of the material at the axis upward to escape speed, forming a jet collimated by an azimuthal field. Finally, material near the base of the jet is brought close to solid-body rotation by Lorentz forces, the jet ceases, and material subsequently ejected through the disk surface angles away from the axis and enters a magnetocentrifugal flow. Jets are produced over a large range in injection and Alfvén speeds, while the magnetocentrifugal flows reach escape speed only when mass flux per field line is low. This mechanism may be useful in explaining the speeds, variability, and mass flow rates of jets and winds from protostars and T Tauri stars.