Numerical simulations of collapse and fragmentation of magnetized molecular cloud cores - Formation of binary systems

The fragmentation process is studied with 3D nested grid MHD simulations. In the run-away collapse phase, the cloud contracts and forms a disk perpendicular to the local magnetic field lines, even if the initial B-field and the angular momentum are not parallel to each other. Angular momentum is transferred by the magnetic field efficiently in this phase as well as in the adiabatic accretion stage afterward. The disk becomes thinner and thinner. If the oblateness (major to minor axis ratio) at the end of the run-away collapse phase is larger than 4, the disk fragments after the disk becomes adiabatic. Binary outflows are formed after the fragments contract. Another candidate for fragmentation is around the second core. Hydrodynamical simulation confirms that in the second collapse with γ ≃ 1:1 a rotation supported disk is formed, although we have no rotation supported disk during the isothermal first run-away collapse (γ = 1). Because this disk is thin, the disk is expected to fragment.