Water shielding in the terrestrial planet-forming zone: Implication for inner disk organics

TL;DR

This study investigates how water UV-shielding influences chemical compositions in the inner protoplanetary disk, revealing enhanced organic molecules and explaining observed abundances without requiring elevated C/O ratios.

Contribution

The paper expands existing models to include water UV-shielding effects, demonstrating its significant impact on organic molecule abundances in the inner disk.

Findings

Water UV-shielding enhances organic molecules like C2H2, CH4, HCN, CH3CN, and NH3 by over 3 orders of magnitude.

Water UV-shielding causes oxygen to be locked in water, suppressing oxygen-rich species like CO and CO2.

Model predicts CH4, NH3, and NO are observable with JWST.

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 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 arxiv:2204.07108 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 C2H2, CH4, HCN, CH3CN, and NH3 by > 3 orders of magnitude. This is caused by the self-shielding of H2O, which locks oxygen in water. This subsequently results in a suppression of oxygen-containing species like CO and CO2. The increase in C2H2 seen in the model with the inclusion of water UV-shielding allows us to explain the observed C2H2 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 CH4, NH3, and NO are expected to be observable with the James Webb Space Telescope (JWST).