Gravitational collapse in turbulent molecular clouds. II. Magnetohydrodynamical turbulence

Hydrodynamic supersonic turbulence can only prevent local gravitational col lapse if the turbulence is driven on scales smaller than the local Jeans le ngths in the densest regions, which is a very severe requirement (see Paper I). Magnetic fields have been suggested to support molecular clouds either magnetostatically or via magnetohydrodynamic (MHD) waves. Whereas the firs t mechanism would form sheetlike clouds, the second mechanism not only coul d exert a pressure onto the gas counteracting the gravitational forces but could lead to a transfer of turbulent kinetic energy down to smaller spatia l scales via MHD wave interactions. This turbulent magnetic cascade might p rovide sufficient energy at small scales to halt local collapse. We test th is hypothesis with MHD simulations at resolutions up to 256(3) zones done w ith ZEUS-3D. We first derive a resolution criterion for self-gravitating, m agnetized gas: to prevent collapse of magnetostatically supported regions c aused by numerical diffusion, the minimum Jeans length must be resolved by four zones. Resolution of MHD waves increases this requirement to roughly s ix zones. We then find that magnetic fields cannot prevent local collapse u nless they provide magnetostatic support. Weaker magnetic fields do somewha t delay collapse and cause it to occur more uniformly across the supported region in comparison to the hydrodynamical case. However, they still cannot prevent local collapse for much longer than a global free-fall time.