2008A&A...483..831A

We present time dependent chemical models for a dense and warm O-rich gas exposed to a strong, far ultraviolet (FUV) field aimed at exploring the formation of simple organic molecules in the inner regions of protoplanetary disks around T Tauri stars. An up-to-date chemical network is used to compute the evolution of molecular abundances. Reactions of H₂ with small organic radicals such as C₂ and C₂H, which are not included in current astrochemical databases, overcome their moderate activation energies at warm temperatures and become very important for the gas phase synthesis of C-bearing molecules. The photodissociation of CO and release of C triggers the formation of simple organic species such as C₂H₂, HCN, and CH₄. In timescales between 1 and 10⁴-years, depending on the density and FUV field, a steady state is reached in the model in which molecules are continuously photodissociated, but also formed, mainly through gas phase chemical reactions involving H₂. The application of the model to the upper layers of inner protoplanetary disks predicts large gas phase abundances of C₂H₂ and HCN. The implied vertical column densities are as large as several 10¹⁶cm^–2 in the very inner disk (<1AU), in good agreement with the recent infrared observations of warm C₂H₂ and HCN in the inner regions of IRS 46 and GV Tau disks. We also compare our results with previous chemical models studying the photoprocessing in the outer disk regions, and find that the gas phase chemical composition in the upper layers of the inner terrestrial zone (a few AU) is predicted to be substantially different from that in the upper layers of the outer disk (>50AU).