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

The Cosmic-Ray Dominated Region of Protoplanetary Disks

We investigate the chemical evolution in the midplane of protoplanetary disks in the region 1 AU ≤ r ≤ 10 AU, 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 O2 as 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

Cosmic Rays and the Origin of Volatiles in Protoplanetary Disks

The origin of water and other volatiles in protoplanetary disks can be either interstellar or due to chemical processing during the protoplanetary disk phase. Depending on the strength of the ionization field present during this stage, an active chemical evolution in the protoplanetary disk midplane can lead to formation of complex volatiles on timescales shorter than the disk dissipation timescale. For this reason, we investigate the effects of cosmic rays and the usually neglected cosmic ray induced UV ionization field in time dependent chemical models of protoplanetary disks. These results are benchmarked against our current knowledge of the chemical composition of cometary ices. We conclude that water and other, more complex volatiles can be preserved in the ice mantles of dust grains. This ice mantle growth can also have a significant impact on the dust opacity and hence on the temperature profile of the disk midplane. This effect will be observable in the near future with ALMA

FORMATION MODELS OF COMETARY ICES IN PROTOPLANETARY DISKS

We set out to constrain the chemical conditions in the early Solar System by analyzing chemical evolution models of protoplanetary disks and comparing them to our current knowledge of Solar System bodies, such as comets. We propose that the region located at 10 AU≤r≤30 AU is ideal for the formation of ice mantles that match observed cometary abundances. The growth of an ice mantle contributes to increase significantly the size of dust grains, which will impact the midplane temperature and the efficiency of dust coagulation