Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
F. Heitsch, M. M. Mac Low, R. S. Klessen;
ApJ, 2001, 547, 280
ABSTRACT:Hydrodynamic supersonic turbulence can only prevent local gravitational collapse if the turbulence is driven on scales smaller than the local
Jeans lengths 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 first mechanism would form sheetlike clouds, the second mechanism not
only could exert a pressure onto the gas counteracting the gravitational
forces but could lead to a transfer of turbulent kinetic energy down to smaller
spatial scales via MHD wave interactions.
This turbulent magnetic cascade
might provide sufficient energy at small scales to halt local collapse.
We
test this hypothesis with MHD simulations at resolutions up to
2563 zones done with ZEUS-3D.
We first derive a resolution criterion for
self-gravitating, magnetized gas: to prevent collapse of magnetostatically supported
regions caused by numerical diffusion, the minimum Jeans length must be
resolved by four zones.
Resolution of MHD waves increases this requirement to
roughly six zones.
We then find that magnetic fields cannot prevent local
collapse unless they provide magnetostatic support.
Weaker magnetic fields
do somewhat 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.
KEYWORDS: ism: clouds, ism: kinematics and dynamics, ism: magnetic fields, turbulence, magnetohydrodynamics: mhd
PERSOKEY:turbulence, ,
CODE: heitsch2001