Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence

Abstract

Hydrodynamic supersonic turbulence can only prevent local gravitational\ncollapse if the turbulence is driven on scales smaller than the local Jeans\nlengths in the densest regions, a very severe requirement (Paper I). Magnetic\nfields have been suggested to support molecular clouds either magnetostatically\nor via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form\nsheet-like clouds, the second mechanism not only could exert a pressure onto\nthe gas counteracting the gravitational forces, but could lead to a transfer of\nturbulent kinetic energy down to smaller spatial scales via MHD wave\ninteractions. This turbulent magnetic cascade might provide sufficient energy\nat small scales to halt local collapse.\n We test this hypothesis with MHD simulations at resolutions up to 256^3\nzones, done with ZEUS-3D. We first derive a resolution criterion for\nself-gravitating, magnetized gas: in order to prevent collapse of\nmagnetostatically supported regions due to numerical diffusion, the minimum\nJeans length must be resolved by four zones. Resolution of MHD waves increases\nthis requirement to roughly six zones. We then find that magnetic fields cannot\nprevent local collapse unless they provide magnetostatic support. Weaker\nmagnetic fields do somewhat delay collapse and cause it to occur more uniformly\nacross the supported region in comparison to the hydrodynamical case. However,\nthey still cannot prevent local collapse for much longer than a global\nfree-fall time.\n