The cosmic-ray dominated region of protoplanetary disks

Abstract

We investigate the chemical evolution in the midplane of protoplanetary disks in the region 1AU≤ r ≤ 10AU, focusing on cosmic ray induced processes. These processes drive the chemical pathways of formation of gas phase molecules which later can be adsorbed onto the surface of grains. We improve on previously existing chemical models by treating the interaction of cosmic rays with the gas/grain environment in a way that is consistent with the local conditions. This means including the effects of dust aggregation in the disk and the extinction of cosmic ray induced UV photons by the gas. We conclude that the effects of cosmic ray UV flux enhancement brought about by grain growth are as relevant as their extinction by gas species. Thus we identify CO, CO2, SiO, S and O2as the main species that contribute to the gas extinction in these regions. The implementation of this method seeks to complete other models that use steady state estimations of the chemical composition of the disk.