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Peter Williams (University of Florida)
The presence of jets in protostellar systems is a surprising observational result. It has been conjectured that the existence of jets is intimately connected with the spin of the central object and the viscous angular momentum transport of the inner disk. We offer a possible mechanism for driving jets from the pressure of small-scale tangled magnetic fields. We propose a viscoelastic model for the effective viscous stress in shear-driven turbulent MHD, in which the small-scale Maxwell stresses are modeled by the addition of a non-Newtonian viscosity, which creates forces that are qualitatively different from a simple effective Newtonian viscosity. These additional forces can have significant qualitative effects. It is known that viscoelastic stresses in the laboratory in azimuthal shear flow generate radially-inwards cylindrical hoop stresses, and the analogous hoop stresses in the astrophysical case are conjectured to drive and collimate the outflow. We discuss this hypothesis in the context of protostellar bipolar outflows.