The role of OH in the chemical evolution of protoplanetary disks II.
  Gas-rich environments
  Context. We present a method for including gas extinction of
cosmic-ray-generated UV photons in chemical models of the midplane of
protoplanetary disks, focusing on its implications on ice formation and
chemical evolution. Aims. Our goal is to improve on chemical models by treating
cosmic rays, the main source of ionization in the midplane of the disk, in a
way that is consistent with current knowledge of the gas and grain environment
present in those regions. We trace the effects of cosmic rays by identifying
the main chemical reaction channels and also the main contributors to the gas
opacity to cosmic-ray-induced UV photons. This information is crucial in
implementing gas opacities for cosmic-ray-induced reactions in full 2D
protoplanetary disk models. Methods. We considered time-dependent chemical
models within the range 1-10 AU in the midplane of a T Tauri disk. The
extinction of cosmic-ray-induced UV photons by gaseous species was included in
the calculation of photorates at each timestep. We integrated the ionization
and dissociation cross sections of all atoms/molecules over the
cosmic-ray-induced UV emission spectrum of H_2. By analyzing the relative
contribution of each gas phase species over time, we were able to identify the
main contributors to the gas opacity in the midplane of protoplanetary disks.
Results. At 1 AU the gas opacity contributes up to 28.2% of the total opacity,
including the dust contribution. At 3-5 AU the gas contribution is 14.5% of the
total opacity, and at 7-8 AU it reaches a value of 12.2%. As expected, at 10-15
AU freeze-out of species causes the gas contribution to the total opacity to be
very low (6%). The main contributors to the gas opacity are CO, CO_2, S, SiO,
and O_2. OH also contributes to the gas opacity, but only at 10-15 AU.
