Clumpy and fractal shocks, and the generation of a velocity dispersion in molecular clouds

doi:10.1111/j.1365-2966.2006.11227.x

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Title:		Clumpy and fractal shocks, and the generation of a velocity dispersion in molecular clouds
Authors:		Dobbs, C. L.; Bonnell, I. A.
Affiliation:		AA(School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS; School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL), AB(School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 374, Issue 3, pp. 1115-1124. (MNRAS Homepage)
Publication Date:		01/2007
Origin:		MNRAS
Astronomy Keywords:		hydrodynamics, turbulence, ISM: clouds, ISM: kinematics and dynamics
DOI:		10.1111/j.1365-2966.2006.11227.x
Bibliographic Code:		2007MNRAS.374.1115D

Abstract

We present an alternative explanation for the nature of turbulence in molecular clouds. Often associated with classical models of turbulence, we instead interpret the observed gas dynamics as random motions, induced when clumpy gas is subject to a shock. From simulations of shocks, we show that a supersonic velocity dispersion occurs in the shocked gas, provided the initial distribution of the gas is sufficiently non-uniform. We investigate the velocity-size-scale relation σ ~ r^α for simulations of clumpy and fractal gas, and show that clumpy shocks can produce realistic velocity-size-scale relations with mean α ~ 0.5. For a fractal distribution, with a fractal dimension of 2.2 similar to what is observed in the interstellar medium, we find σ ~ r^0.4. The form of the velocity-size-scale relation can be understood as due to mass-loading, that is, the post-shock velocity of the gas is determined by the amount of mass encountered as the gas enters the shock. We support this hypothesis with analytical calculations of the velocity dispersion relation for different initial distributions. A prediction of this model is that the line-of-sight velocity dispersion should depend on the angle at which the shocked gas is viewed.

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