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Turbulence in Interstellar Clouds

Published online by Cambridge University Press:  12 April 2016

E. Falgarone
E. Falgarone*
Affiliation:
California Institute of Technology, 405-47, Pasadena, CA91125, USA Radioastronomie, Ecole Normale Supérieure, 24 rue Lhomond, 75235 Paris Cedex 05, France

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Above masses of the order of lOO M⊙, molecular clouds have masses and sizes which scale like those of self-gravitating polytropes bounded by an external pervading pressure. It is unlikely that this scaling is due to mere observational bias. But the physics underlying this behaviour is far from being understood. In particular, the possible contribution of turbulence to both the ambient pressure and the internal pressure (whose dependence with the density would mimic a polytropic behavior) is a difficult and much debated issue. The clouds mass, size and internal velocity dispersion are such that they are observed to be in approximate virial balance between their self-gravity, the surface energy term due to the ambient pressure and their internal energy. The latter is dominated by the kinetic energy of disordered internal motions. However, there has been little evidence so far that these motions are actually turbulent rather than simply disordered. The transition to turbulence in a flow occurs when the non linear advection term in the momentum equation, v.Δv, considerably exceeds the viscous dissipation term, vΔv (where v is the kinematic viscosity). Non linearities therefore dominate the physics of a turbulent flow and the velocities are not randomly distributed. Most of the previous attempts to determine a well-defined correlation length in the velocity field (Kleiner and Dickman 1985, a and b; Scalo 1984), which is predicted to be close to the scale at which the energy is injected, or to characterize the expected hierarchical structure (Pérault et al. 1986) have been plagued by the lack of dynamical range in the data set and the range of scales over which the correlation functions have been computed. The most plausible determination, that of Kleiner and Dickman (1987) who claim to have found a correlation length of 0.2 pc in the Taurus cloud, gives a result which is so close to the angular resolution of the observations that it is doubtful.

Type
I. Molecular Clouds, Star Formation And HII Regions
Information
International Astronomical Union Colloquium , Volume 120: Structure and Dynamics of the Interstellar Medium , 1989 , pp. 68 - 79
Copyright
Copyright © Springer-Verlag 1989

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