Transport of Dust Grains in Turbulent Molecular Clouds

Abstract

Recently, F. P. Pijpers has estimated anew the average residence time of dust grains in turbulent molecular clouds, modeling the grains' dynamics as a random walk along turbulent eddies of a Kolmogorov energy spectrum, with distribution versus size similar to that of the clumps. Implicitly, he assumed that this results in a diffusive transport process (the spreading of the dust grains Δr2(t) increasing linearly with time t), and obtained a residence time scaling almost as the Reynolds number of the flow, and hence much longer than previous estimates. Following the same random walk model, the grains are shown here to be transported anomalously quickly (Δr2(t) ∝ tα with α ≥ 3), and a much shorter time is found, scaling as the Reynolds number to some negative power. This might indicate the need to take into account the organization of the turbulent structures when computing the transport exponent α.