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P. Williams (U. Texas)
We have argued that the stress due to small-scale magnetic fields, as generated by the Balbus-Hawley instability, may generate sufficient hoop stresses within a thick disk to drive and collimate outflows (jets). Such a driving and collimation mechanism would be complimentary to theories in which jets are driven by large-scale, organized magetic fields. In contrast, we note that, in the language of mean field theory, the mean-field stress tensor may be substantial even in the case that the mean field itself is zero or negligible. In particular, the nonlinear saturation of the Balbus-Hawley instability can result in Maxwell stresses which are of the correct order of magnitude to explain the anomalous viscosity of accretion disks, even in the case that the initial seed field is relatively small. The interpretation of these stresses as an effective viscosity obscures the fact that these stresses have a finite relaxation time and focuses attention on a single component (the r-theta component) of the stress tensor, when in fact on-diagonal components of the viscous stress tensor may also be substantial, in contrast to the behavior of a simple Newtonian viscous fluid. Such stresses, we have postulated, may cause outflows.
Here we address the predictions of a simple model for jet acceleration by these stresses. We discuss a few simple ``viscoelastic" models for the turbulent Maxwell stresses, which extend the simple Shakura-Sunyaev effective viscous stress prescription, and focus our attention on the predictions of these models for the viscous coupling between disk, jet, and star in protostellar systems. We explore the connection between jet acceleration and both the magnitude and the direction of the angular momentum of the central star, and their effect on the magnitude and direction of the jets. The extension of the model for the effective stress tensor in a thick disk beyond a simple Newtonian viscosity has broad implications. We demonstrate that, among other results, the finite relaxation time incorprated into such models can cause precession of the central star when it is viscously coupled to the disk. We discuss the predictions of these models for the precession frequency of the central star in the context of observations of jet precession in protostellar systems.
The author(s) of this abstract have provided an email address for comments about the abstract: petwil@astro.as.utexas.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.