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

We show in this Letter that the spectral details of the 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 Lyman α radiation. The effects of the strong Lyman α emission on the chemical equilibrium in protoplanetary disks has 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, Lyman α radiation will directly dissociate water vapor and may contribute to the observed enhancements of CN/HCN in disks. Subject headings: accretion, accretion disks, astrobiology, astrochemistry — circumstellar matter — stars: pre-main sequence — ultraviolet:stars