Turbulence-driven Diffusion in Protoplanetary Disks: Chemical Effects in the Outer Regions

The chemistry and dynamics of protoplanetary disks are likely to be intricately linked, with dynamical processes affecting the chemical composition and chemistry, in turn, controlling the ionization structure and hence the ability of a process such as the magnetorotational instability to drive turbulence. Here we present the results from chemical models of the outer disk, which include diffusive mixing driven by turbulence. We show that diffusion in the vertical direction can greatly affect the column densities of many molecules, increasing them by up to 2 orders of magnitude. Previous models have shown that disks consist of three chemically distinct layers, with the bulk of the observed molecular emission coming from a region between the cold midplane and the irradiated surface layers. Diffusion retains this structure, but increases the depth of the molecular layer, by bringing atoms and atomic ions formed by photodissociation in the layers into more shielded regions where the molecules can reform. The column densities of molecules whose abundances peak closer to the midplane, e.g., NH3 and N2H+, are not altered by diffusion. We find that diffusion does not affect the ionization fraction of the disk. We compare the calculated column densities to observations of DM Tau, LkCa 15, and TW Hya and find good agreement for many molecules for models with a diffusion coefficient of 1018 cm2 s-1.