Turbulence driven diffusion in protoplanetary disks - chemical effects in the outer disk

Abstract

The dynamics and chemistry of protostellar disks are likely to be intricately linked, with dynamical processes altering the chemical composition, and chemistry, in turn, controlling the ionization structure and hence the ability of the magneto-rotational instability to drive the disk turbulence. Here we present the results from the first chemical models of the outer regions (R > 100 AU) of protoplanetary disks to consider the effects of turbulence driven diffusive mixing in the vertical direction. We show that vertical diffusion can greatly affect the column densities of many species, increasing them by factors of up to two orders of magnitude. Previous disk models have shown that disks can be divided into three chemically distinct layers, with the bulk of the observed molecular emission coming from a region between an atomic/ionic layer on the surface of the disk and the midplane regoin where the bulk of molecules are frozen onto grains. Diffusion retains this three layer structure, but increases the depth of the molecular layer by bringing atoms and atomic ions form by photodissociation in the surface layers into the shielded molecular layer where molecules can reform. For other species, notably NH3 and N2H+, the column densities are relatively unaffected by diffusion. These species peak in abundance near the midplane where most other molecules are heavily depleted, rather than in the molecular layer above. Diffusion only affects the abundances of those molecules with peak abundances in the molecular layer. 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 with a diffusion coefficient of 1e18 cm^2 s^-1.