Formation and loss of hierarchical structure in two-dimensional MHD simulations of wave-driven turbulence in interstellar clouds

Abstract

Two dimensional compressible magneto-hydrodynamical (MHD) simulations run for 20 crossing times on a 800x640 grid with two stable thermal states show persistent hierarchical density structures and Kolmogorov turbulent motions in the interaction zone between incoming non-linear Alfven waves. These structures and motions are similar to what are commonly observed in weakly self-gravitating interstellar clouds, suggesting that these clouds get their fractal structures from non-linear magnetic waves generated in the intercloud medium; no internal source of turbulent energy is necessary. The clumps in the simulated clouds are slightly warmer than the interclump medium as a result of magnetic dissipational and compressive heating. Thus the interclump medium has a lower pressure than the clumps, demonstrating that the clumps owe their existence to transient compressive motions, not pressure confinement. Clump lifetimes increase with size, and are about one sound crossing time. Two specific problems are addressed. We find that low ionization fractions in molecular clouds do not necessarily lead to increased cloud smoothing, but that enhanced density in a self-gravitating cloud does, because of external wave self-shielding. Bulk cloud self-gravity may therefore promote star formation in the relatively quiescent gas pools that contain a thermal Jeans mass or more.