CHEMISTRY OF A PROTOPLANETARY DISK WITH GRAIN SETTLING AND Ly alpha RADIATION

We present results from a model of the chemical evolution of protoplanetary disks. In our models, we directly calculate the changing propagation and penetration of a high energy radiation field with Lya radiation included. We also explore the effect on our models of including dust grain settling. We find that, in agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and a correspondingly larger molecular layer, which leads to an increase in column density for molecular species such as CO, CN, and SO. The inclusion of Lya radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216 angstrom. These include HCN, NH3, and CH4, for which the column densities are decreased by an order of magnitude or more due to the presence of Ly alpha radiation in the UV spectrum. A few species, such as CO2 and SO, are enhanced by the presence of Ly alpha radiation, but rarely by more than a factor of a few.