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Dense, star-forming, cores of molecular clouds are observed to besignificantly magnetized. A realistic magnetic field of moderate strength hasbeen shown to suppress, through catastrophic magnetic braking, the formation ofa rotationally supported disk during the protostellar accretion phase oflow-mass star formation in the ideal MHD limit. We address, through 2D(axisymmetric) simulations, the question of whether realistic levels ofnonideal effects, computed with a simplified chemical network including dustgrains, can weaken the magnetic braking enough to enable a rotationallysupported disk to form. We find that ambipolar diffusion, the dominant nonidealMHD effect over most of the density range relevant to disk formation, does notenable disk formation, at least in 2D. The reason is that ambipolar diffusionallows the magnetic flux that would be dragged into the central stellar objectin the ideal MHD limit to pile up instead in a small circumstellar region,where the magnetic field strength (and thus the braking efficiency) is greatlyenhanced. We also find that, on the scale of tens of AU or more, a realisticlevel of Ohmic dissipation does not weaken the magnetic braking enough for arotationally supported disk to form, either by itself or in combination withambipolar diffusion. The Hall effect, the least explored of these threenonideal MHD effects, can spin up the material close to the central object to asignificant, supersonic rotation speed, even when the core is initiallynon-rotating, although the spun-up material remains too sub-Keplerian to form arotationally supported disk. The problem of catastrophic magnetic braking thatprevents disk formation in dense cores magnetized to realistic levels remainsunresolved. Possible resolutions of this problem are discussed.

Nonideal MHD Effects and Magnetic Braking Catastrophe in Protostellar Disk Formation