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P.E. Hardee (U. Alabama), R.C. Walker (NRAO/VLA), J.-L. Gomez (Inst. Astrophys. Andalucia)
Where superluminal motions are detected on an expanding jet, modeling efforts based on an observed helical twist can be used to determine the jet sound speed and the sound speed in the immediately surrounding environment. This can be accomplished as the spatial development of a helical twist depends on the wave frequency relative to a local ``resonant'' frequency, \omega ~1.5 ax/Rj, which decreases along an expanding jet as the jet radius, Rj, increases and the external sound speed, ax, decreases. The wave speed is different and depends differently on jet and sound speeds in the ``low'' and ``high'' frequency regimes. Observed change in wavelength and pattern speed implies an evolution from a low frequency to high frequency regime. Lack of change implies a twist in either the low or high frequency regime. In general, well developed helical structure on a highly collimated jet implies a ``saturated'' structure in the high frequency regime. These clues can be used to identify the frequency regime(s) in which an observed twist resides.
The determination of sound speeds requires observed proper motions for both the pattern speed and the jet speed. We illustrate the basic intrinsic and observed behavior of a helically twisted structure and use the observed structure and motions in the inner 15~mas of the 3C\,120 jet to find sound speeds in the jet and in the external medium.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.