The role of OH in the chemical evolution of protoplanetary disks:I. The comet-forming region

Context. Time-dependent gas-grain chemistry can help us understand the layered structure of species deposited onto the surface of grains during the lifetime of a protoplanetary disk. The history of trapping large quantities of carbon-and oxygen-bearing molecules onto the grains is especially significant for the formation of more complex (organic) molecules on the surface of grains.Aims. Among other processes, cosmic ray-induced UV photoprocesses can lead to the efficient formation of OH. Using a more accurate treatment of cosmic ray-gas interactions for disks, we obtain an increased cosmic ray-induced UV photon flux of 3.8 x 10(5) photons cm(-2) s(-1) for a cosmic-ray ionization rate of H-2 value of 5 x 10(-17) s(-1) (compared to previous estimates of 10(4) photons cm(-2) s(-1) based on ISM dust properties). We explore the role of the enhanced OH abundance on the gas-grain chemistry in the midplane of the disk at 10 AU, which is a plausible location of comet formation. We focus on studying the formation/destruction pathways and timescales of the dominant chemical species.Methods. We solved the chemical rate equations based on a gas-grain chemical network and correcting for the enhanced cosmic ray-induced UV field. This field was estimated from an appropriate treatment of dust properties in a protoplanetary disk, as opposed to previous estimates that assume an ISM-like grain size distribution. We also explored the chemical effects of photodesorption of water ice into OH+H.Results. Near the end of the disk's lifetime our chemical model yields H2O, CO, CO2, and CH4 ice abundances at 10 AU (consistent with a midplane density of 10(10) cm(-3) and a temperature of 20 K) that are compatible with measurements of the chemical composition of cometary bodies for a [C/O] ratio of 0.16. This comparison puts constraints on the physical conditions in which comets were formed.