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We investigate the roles of magnetic fields and ambipolar diffusion duringprestellar core formation in turbulent giant molecular clouds (GMCs), usingthree-dimensional numerical simulations. Our simulations focus on the shockedlayer produced by a converging flow within a GMC, and survey varying ionizationand angle between the upstream flow and magnetic field. We also include idealmagnetohydrodynamic (MHD) and hydrodynamic models. From our simulations, weidentify hundreds of self-gravitating cores that form within 1 Myr, with massesM ~ 0.04 - 2.5 solar-mass and sizes L ~ 0.015 - 0.07 pc, consistent withobservations of the peak of the core mass function (CMF). Median values are M =0.47 solar-mass and L = 0.03 pc. Core masses and sizes do not depend on eitherthe ionization or upstream magnetic field direction. In contrast, themass-to-magnetic flux ratio does increase with lower ionization, from twice tofour times the critical value. The higher mass-to-flux ratio for low ionizationis the result of enhanced transient ambipolar diffusion when the shocked layerfirst forms. However, ambipolar diffusion is not necessary to form low-masssupercritical cores. For ideal MHD, we find similar masses to other cases.These masses are 1 - 2 orders of magnitude lower than the value that defines amagnetically supercritical sphere under post-shock ambient conditions. Thisdiscrepancy is the result of anisotropic contraction along field lines, whichis clearly evident in both ideal MHD and diffusive simulations. We interpretour numerical findings using a simple scaling argument which suggests thatgravitationally critical core masses will depend on the sound speed and meanturbulent pressure in a cloud, regardless of magnetic effects.

Formation of Magnetized Prestellar Cores with Ambipolar Diffusion and Turbulence