Chondrule Formation and Protoplanetary Disk Heating by Current Sheets in Nonideal Magnetohydrodynamic Turbulence

We study magnetic field steepening due to ambipolar diffusion in protoplanetary disk environments and draw the following conclusions. Current sheets are generated in magnetically active regions of the disk where the ionization fraction is high enough for the magnetorotational instability to operate. In late stages of solar nebula evolution, the surface density is expected to decrease and dust grains are expected to gravitationally settle to the midplane. If the local dust-to-gas mass ratio near the midplane is increased above cosmic abundances by factors ≳103, current sheets reach high enough temperatures to melt millimeter-sized dust grains and hence may provide the mechanism to form meteoritic chondrules. In addition, these current sheets possibly explain the near-infrared excesses observed in spectral energy distributions (SEDs) of young stellar objects. Direct imaging of protoplanetary disks via a nulling interferometer or, in the future, a multiband, adaptive optics coronagraph can test this hypothesis.