Formation of simple organic molecules in inner T Tauri disks

Abstract

Aims. 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. Methods. 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. Results. The photodissociation of CO and release of C triggers the formation of simple organic species such as C2H2 ,H CN, and CH 4. In timescales between 1 and 10 4 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. Conclusions. 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 10 16 cm −2 in the very inner disk ( 50 AU).