Water UV-shielding in the Terrestrial Planet-forming Zone: Implications for Oxygen-18 Isotope Anomalies in H218O Infrared Emission and Meteorites

Abstract

An understanding of abundance and distribution of water vapor in the innermost region of protoplanetary disks is key to understanding the origin of habitable worlds and planetary systems. Past observations have shown H2O to be abundant and a major carrier of elemental oxygen in disk surface layers that lie within the inner few astronomical units of the disk. The combination of high abundance and strong radiative transitions leads to emission lines that are optically thick across the infrared spectral range. Its rarer isotopologue H 2 18 O traces deeper into this layer and will trace the full content of the planet-forming zone. In this work, we explore the relative distribution of H 2 16 O and H 2 18 O within a model that includes water self-shielding from the destructive effects of ultraviolet radiation. In this Letter we show that there is an enhancement in the relative H 2 18 O abundance high up in the warm molecular layer within 0.1–10 au due to self-shielding of CO, C18O, and H2O. Most transitions of H 2 18 O that can be observed with JWST will partially emit from this layer, making it essential to take into account how H2O self-shielding may effect the H2O to H 2 18 O ratio. Additionally, this reservoir of H 2 18 O -enriched gas in combination with the vertical “cold finger” effect might provide a natural mechanism to account for oxygen isotopic anomalies found in meteoritic material in the solar system.

High-contrast JWST-MIRI Spectroscopy of Planet-forming Disks for the JDISC Survey

“…Because of the high dust optical depth of typical inner disks, the infrared emission generally traces the surface, representing 0.1%-1% of the total vertical column density at 1 au (Woitke et al 2018;Bosman et al 2022). These regions are expected to have highly active chemistry, due to the high temperatures and exposure to UV and X-ray radiation from the central star and accretion shock (Semenov & Wiebe 2011;Calahan et al 2022). Consequently, molecular abundances may be strongly altered, by orders of magnitude, from their primordial composition (Pontoppidan et al 2014).…”

Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the Benchmark HIP 55507 AB System

“…In our baseline retrievals, we fit for three ratios: 18 . This allows us to explore whether the 16 O/ 18 O ratio differs between CO and H 2 O, which may arise from isotopic fractionation processes such as self-shielding of CO and UV shielding of H 2 O (Calahan et al 2022). In addition, fitting two ratios allows us to examine whether the data show evidence for H O 2 18 , 12 C 18 O, or both.…”

The Implications of Thermal Hydrodynamic Atmospheric Escape on the TRAPPIST-1 Planets

“…In reality, photochemical reactions play a pivotal role in atmospheric escape, particularly in UV shielding, the creation of photochemical products that may act as UV shields, and effects on atmospheric heating and cooling. Through UV shielding, particles at higher altitudes may absorb UV radiation and protect molecules lower in the atmosphere from photodissociation, including self-shielding (e.g., O 2 protecting other O 2 molecules) and shielding between molecular species (Calahan et al 2022). Of notable importance, the absorption cross sections of water, O 2 , O 3 , and CO 2 overlap heavily (Parkinson & Yoshino 2003;Domagal-Goldman et al 2014); thus, the intense buildup of O 2 we observe in our models may serve to protect water from photodissociation, lowering massloss rates.…”