**AAS 196th Meeting, June 2000**

*Session 26. The Galactic ISM: Observations and Modelling*

Display, Tuesday, June 6, 2000, 10:00am-6:30pm, Empire Hall South
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## [26.03] Characterization of Supersonic, MHD Turbulence in Models of Molecular Clouds

*M.- M. Mac Low (AMNH), V. Ossenkopf (U. Cologne), M. D. Smith (Armagh Obs.), J. M. Zuev (U. Colo. at Boulder), F. Heitsch (MPIfA)*

Strongly supersonic, mildly super-Alfvénic turbulence has
been proposed to be the best model for the structure of
star-forming molecular clouds. We characterize 3D numerical
models of decaying and uniformly driven, isothermal,
supersonic, super-Alfénic turbulence in order to compare
it with observations of molecular clouds and determine the
properties of the observed turbulence.

We study a number of different statistical descriptions of
the turbulence. The \Delta-variance proposed by Stutzki et
al.\ is an averaged wavelet transform revealing structure at
different scales. Observations tend to show power-law,
self-similar behavior for the \Delta-variance, which we
can reproduce only with strongly supersonic, weakly
magnetized models driven from the largest available scales.
The shapes of velocity difference probability distribution
functions also only agree with observations in the case of
large-scale driving and weak magnetic fields.

Study of the distribution of shock velocities in the
turbulence reveals that decaying turbulence differs markedly
from continuously driven turbulence. We find decaying
turbulence to have an exponential tail of high-velocity
shocks, with the number of shocks per velocity interval over
time t excellently fit by the function t \exp (-ktv) for
shock jump v and initial driving wavenumber k. This form
can be derived from an analytic extension of mapping closure
techniques. Driven turbulence, on the other hand, has an
inverse square-root distribution of high-velocity shocks.
The power dissipation per unit velocity can be readily
derived for both of these types of turbulence, enabling
direct comparison to observations. Inclusion of self-gravity
adds a number of strong, dissipative accretion shocks, but
does not change the overall behavior.

The author(s) of this abstract have provided an email address
for comments about the abstract:
mordecai@amnh.org

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