Collapse and Fragmentation of Molecular Cloud Cores. IX. Magnetic Braking of Initially Filamentary Clouds

The collapse and fragmentation of initially filamentary, magnetic molecular clouds is calculated in three dimensions with a gravitational, radiative hydrodynamics code. The code includes magnetic field effects in an approximate manner: magnetic pressure, tension, braking, and ambipolar diffusion are all modelled. Three types of outcomes are observed: direct collapse and fragmentation into a multiple protostar system, periodic contraction and expansion without collapse, or periodic contraction and expansion leading eventually to collapse. While the models begin their evolution at rest except for the assumed solid-body rotation, they develop weakly supersonic velocity fields as a result of the rebounding prior to collapse. The models show that magnetically-supported clouds subject to magnetic braking can undergo dynamic collapse leading to protostellar fragmentation on scales of 10 AU to 100 AU, consistent with typical binary star separations.