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Requirements for magnetic coupling and accretion in the active layer of aprotostellar disk are re-examined, and some implications for thermal emissionfrom the layer are discussed. The ionization and electrical conductivity arecalculated following the general scheme of Ilgner and Nelson but with anupdated UMIST database of chemical reactions and some improvements in the grainphysics, and for the minimum-mass solar nebula rather than an alpha disk. Thenew limits on grain abundance are slightly more severe than theirs. Even foroptimally sized grains, the layer should be at least marginally optically thinto its own thermal radiation, so that narrow, highly saturated emission linesof water and other molecular species would be expected if accretion is drivenby turbulence and standard rates of ionization prevail. If the grain sizedistribution extends broadly from well below a micron to a millimeter or more,as suggested by observations, then the layer may be so optically thin that itscooling is dominated by molecular emission. Even under such conditions, it isdifficult to have active layers of more than 10g/cm^2 near 1AU unless dust isentirely eliminated or greatly enhanced ionization rates are assumed.Equipartition-strength magnetic fields are then required in these regions ofthe disk if observed accretion rates are driven by magnetorotationalturbulence. Wind-driven accretion might allow weaker fields and less massiveactive layers but would not heat the layer as much as turbulence and thereforemight not produce emission lines.

Heat and Dust in Active Layers of Protostellar Disks