The Impact of External Radiation on the Inner Disk Chemistry of Planet Formation

Abstract The vast majority of young stars hosting planet-forming disks exist within clustered environments, like the Orion Nebula, implying that seemingly “extreme” UV environments (10 4 G 0 and above) are not so atypical in the context of planet formation. Using thermochemical modeling, we explore how the temperature and chemistry within a protoplanetary disk around a T Tauri star are impacted by the surrounding UV environment. The disk becomes hotter due to heating by photodissociation of molecules, photoelectric heating, H 2 , and atomic processes, and as a result, the area in which molecules exist in the ice phase shrinks, being pushed both downward and inward. Beyond 1 au, the chemistry changes most significantly in a UV-rich background; the atmosphere becomes more H 2 O, OH, and atomic-rich. Hydrocarbons, however, reside primarily well within 1 au of the disk; thus, their abundance and distribution are not impacted by the UV field up to 10 6 G 0 . The products of photodissociation and photochemistry are formed deeper into the disk with increasing UV background field strength beyond 1 au, impacting the chemistry near the midplane. Effectively, a “reset” chemistry takes place, with an enhancement of atoms, simple molecules, and molecules in the gas phase. Planets that form in highly irradiated regions will be exposed to a different chemical reservoir in the gas and ice phases than that in an isolated disk, and the impact from the UV background should only be detectable in highly irradiated disks (∼10 6 G 0 ).