Water Shielding in the Terrestrial Planet-forming Zone: Implication for Inner Disk Organics

Abstract

The chemical composition of the inner region of protoplanetary disks can trace the composition of planetary-building material. The exact elemental composition of the inner disk has not yet been measured and tensions between models and observations still exist. Recent advancements have shown UV shielding to be able to increase the emission of organics. Here, we expand on these models and investigate how UV shielding may impact chemical composition in the inner 5 au. In this work, we use the model from Bosman et al. and expand it with a larger chemical network. We focus on the chemical abundances in the upper disk atmosphere where the effects of water UV shielding are most prominent and molecular lines originate. We find rich carbon and nitrogen chemistry with enhanced abundances of C 2 H 2 , CH 4 , HCN, CH 3 CN, and NH 3 by >3 orders of magnitude. This is caused by the self-shielding of H 2 O, which locks oxygen in water. This subsequently results in a suppression of oxygen-containing species like CO and CO 2 . The increase in C 2 H 2 seen in the model with the inclusion of water UV shielding allows us to explain the observed C 2 H 2 abundance without resorting to elevated C/O ratios as water UV shielding induced an effectively oxygen-poor environment in oxygen-rich gas. Thus, water UV shielding is important for reproducing the observed abundances of hydrocarbons and nitriles. From our model result, species like CH 4 , NH 3 , and NO are expected to be observable with the James Webb Space Telescope (JWST).