The importance of photoprocessing in protoplanetary disks

Midplane models of protoplanetary disks iind that the cold temperatures in the outer parts of the disk ensure that virtually all molecules are accrete d onto the grains. However, molecules in the gas are observed at these radi i. One possible explanation is that the emission arises from above the midp lane, possibly in a heated layer at the surface of a flared disk. Models wh ich take into account the vertical chemical distribution of molecules and c an calculate column densities are therefore required for comparison with ob servations. We present the results of a calculation of the time-dependent t wo-dimensional chemical structure of a flared protoplanetary disk which inc ludes photoprocesses driven by both the stellar and interstellar radiation fields. Three layers are found in the disk consistent with previous work. I n the upper layer photodissociation produces large abundances of atoms and ions. Below this molecules are shielded and can avoid dissociation, althoug h sufficient radiation is present to remove molecules from the grain surfac es by photodesorption. The majority of the observable species come from thi s layer. Closer to the midplane of the disk, freezeout removes molecules fr om the gas. We find that photodesorption can account for the observed colum n densities if the photodesorption yield is higher than 10(-3) molecules pe r photon. These results indicate that many observed molecules trace the phy sical and chemical conditions in the surface regions rather than the midpla ne although the contribution of the heated surface layer to the column dens ities is minimal.