Abstract
We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. We found that a snapshot of the relation between density (ρ) and the strength of the magnetic field (B) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both the relations tend to B ∝ ρ0.5. We also have demonstrated that the formation of collapsing cores in subcritical clouds is accelerated by the supersonic nonlinear flows. Although the time-scale of the core formation in subcritical clouds is normally estimated to be a few ×10^7 years, we found that it is shortened to approximately several ×10^6 years by the supersonic flows. The result is consistent with previous two-dimensional simulations.
