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

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 ${{\rm{H}}}_{2}^{18}{\rm{O}}$ 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 ${{\rm{H}}}_{2}^{16}{\rm{O}}$ and ${{\rm{H}}}_{2}^{18}{\rm{O}}$ 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 ${{\rm{H}}}_{2}^{18}{\rm{O}}$ 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 ${{\rm{H}}}_{2}^{18}{\rm{O}}$ 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 ${{\rm{H}}}_{2}^{18}{\rm{O}}$ ratio. Additionally, this reservoir of ${{\rm{H}}}_{2}^{18}{\rm{O}}$-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.