Nonideal MHD Effects and Magnetic Braking Catastrophe in Protostellar Disk Formation

Abstract

Dense, star-forming, cores of molecular clouds are observed to be significantly magnetized. A realistic magnetic field of moderate strength has been shown to suppress, through catastrophic magnetic braking, the formation of a rotationally supported disk during the protostellar accretion phase of low-mass star formation in the ideal MHD limit. We address, through 2D (axisymmetric) simulations, the question of whether realistic levels of nonideal effects, computed with a simplified chemical network including dust grains, can weaken the magnetic braking enough to enable a rotationally supported disk to form. We find that ambipolar diffusion, the dominant nonideal MHD effect over most of the density range relevant to disk formation, does not enable disk formation, at least in 2D. The reason is that ambipolar diffusion allows the magnetic flux that would be dragged into the central stellar object in the ideal MHD limit to pile up instead in a small circumstellar region, where the magnetic field strength (and thus the braking efficiency) is greatly enhanced. We also find that, on the scale of tens of AU or more, a realistic level of Ohmic dissipation does not weaken the magnetic braking enough for a rotationally supported disk to form, either by itself or in combination with ambipolar diffusion. The Hall effect, the least explored of these three nonideal MHD effects, can spin up the material close to the central object to a significant, supersonic rotation speed, even when the core is initially non-rotating, although the spun-up material remains too sub-Keplerian to form a rotationally supported disk. The problem of catastrophic magnetic braking that prevents disk formation in dense cores magnetized to realistic levels remains unresolved. Possible resolutions of this problem are discussed.