[Previous] | [Session 93] | [Next]
P. Williams (Los Alamos National Laboratory)
It is often assumed in studies of jet launching that there is a transonic surface separating the supersonic jet from the presumably subsonic launching region. This is true in older purely hydrodynamic jet mechanisms in which there is a de Laval nozzle, as well as in magnetocentrifugal mechanisms, although in the latter case there is more than one type of transonic surface, corresponding to the different wave modes in magnetohydrodyanamics. We note that both jets and thin disks are highly supersonic, and we hypothesize that in a transitional accretion flow that connects a thin disk to a jet, the fluid that ends up in the jet might actually nowhere be subsonic.
If the fluid that ends up in the jet (henceforth ``working fluid'') is everywhere supersonic, then ram pressure should be significant where the velocity transitions from the azimuthal disk motion to the axial motion in a jet. This ram pressure should help collimate the outflow in the innermost regions of the jet-launching region. We note that ram pressure is an effective way to collimate and drive jets in the laboratory.
We previously hypothesized that turbulence in the accretion flow (as driven by the magnetorotational instability or by convection) naturally generates hoop-stresses, in analogy to the azimuthal shear flow of a viscoelastic fluid, and that these stresses also help to drive and collimate the outflow. These stresses may be critical to keeping the accretion flow supersonic in the transition region.
We assume that the flow of the working fluid is thermally coupled to the large amount of energy released in the central accretion region. The latter would thus behave as a ``furnace,'' so that the working fluid would be compressed, heated, and allowed to expand again. We thus hypothesize that protostellar jets might be analogous to supersonic ramjets, or scramjets.
The author(s) of this abstract have provided an email address for comments about the abstract: petwil@astro.as.utexas.edu
[Previous] | [Session 93] | [Next]
Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.