The Effects of UV Continuum and Lyα Radiation on the Chemical Equilibrium of T Tauri Disks

We show in this Letter that the spectral details of the far-ultraviolet (FUV) radiation fields have a large impact on the chemistry of protoplanetary disks surrounding T Tauri stars. We show that the strength of a realistic stellar FUV field is significantly lower than typically assumed in chemical calculations and that the radiation field is dominated by strong line emission, most notably Lyα radiation. The effects of the strong Lyα emission on the chemical equilibrium in protoplanetary disks have previously been unrecognized. We discuss the impact of this radiation on molecular observations in the context of a radiative transfer model that includes both direct attenuation and scattering. In particular, Lyα radiation will directly dissociate water vapor and may contribute to the observed enhancements of CN/HCN in disks.