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Session 20 - Chemistry & Physical Process in the ISM.
Display session, Wednesday, January 07
Exhibit Hall,

[20.02] Simulations of MHD Turbulence in Molecular Clouds: Decay Timescales and Spatial Structure

M. -M. Mac Low, R. Klessen, A. Burkert (MPIA), M. D. Smith (U. Würzburg), O. Kessel (MPIA)

We perform full numerical simulations in 3D of supersonic, super-Alfvénic, isothermal turbulence, using the well-tested magnetohydrodynamic (MHD) code ZEUS-3D, as well as supporting subsonic, sub-Alfvénic, and adiabatic models, supplemented by hydrodynamic models using SPH. We use resolutions as high as 256^3 zones, and 350,000 particles in the SPH models, allowing us to clearly separate dissipation scales from turbulent scales, as demonstrated by resolution studies. We find the surprisingly general result that the kinetic energy of turbulence decays as t^-s, with the decay power-law s in the range 1 < s < 1.3 for all of the regimes we have studied, aside from the extreme subsonic approach to laminar flow. A value of s \sim 1.2-1.3 was already well known for subsonic turbulence; surprisingly it appears to apply as well to the supersonic and MHD turbulence characteristic of molecular clouds. Such a fast decay rate, even in the presence of magnetic fields, rules out models of molecular clouds that do not include some form of energy input driving the observed turbulence, whether it be galactic shear, blast waves and ionization from massive stars, or jets from low-mass stars. We also compute the fractal dimension of our models for comparison to observations. We find that the usual assumption of the additive law---that a 2D slice through a 3D fractal will have a dimension one less than the original fractal--does not always hold. Rather, we have found cases where a 2D slice or projection comparable to observation actually has the same fractal dimension as the full 3D models. Finally, we attempt to use the fractal dimension to constrain the physical parameters of observed molecular clouds by comparison to our models.

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