UV protection of water in the terrestrial planet formation zone: implications for oxygen-18 isotope anomalies in infrared H2-18O emission and meteorites


July 11, 2022

The abundance of H2O, H18 2 O, CO and C18O in units of (mol/cm−3 )/ngas throughout the inner disk (left and center graphs) and the ratio between H2O, H18 2 O (top right) and CO and C18O (bottom right). At z/r≈ 0.2 a unique region exists where H 18 2 O is increased and C18O is strongly depleted.

An understanding of the abundance and distribution of water vapor in the innermost region of protoplanetary disks is essential to understanding the origin of habitable worlds and planetary systems.

Past observations have shown that H2O is abundant and is a major carrier of elemental oxygen in the surface layers of the disk that lie in the inner few of the disk. The combination of high abundance and strong radiative transitions leads to optically thick emission lines over the entire infrared spectral range. Its rarer isotopologue H2-18O traces deeper into this layer and will trace all the contents of the planet formation zone. In this work, we explore the relative distribution of H2-16O and H2-18O in a model that includes the self-protection of water against the destructive effects of ultraviolet radiation.

In this letter, we show that there is an enhancement of the relative abundance of H2-18O in the hot molecular layer between 0.1 and 10 AU due to self-shielding of CO, C18O and H2O. Most H2-18O transitions that can be observed with JWST will partially emit from this layer, making it essential to consider how self-shielding of H2O can affect the H2O to H2-18O ratio. Moreover, this reservoir of H2-18O-enriched gas in combination with the vertical “cold finger” effect could provide a natural mechanism to explain the oxygen isotope anomalies found in meteoritic materials in the solar system.

Jenny K. Calahan, Edwin A. Bergin, Arthur D. Bosman

Comments: 8 pages, 4 digits, accepted at ApJ Letters
Subjects: Terrestrial and planetary astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2207.04063 [astro-ph.EP] (or arXiv:2207.04063v1 [astro-ph.EP] for this version)
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Submission history
From: Jenny Calahan
[v1] Fri Jul 8, 2022 6:00:02 PM UTC (6,245 KB)
Astrobiology, Astrochemistry,

Co-founder of SpaceRef, member of the Explorers Club, ex-NASA, external teams, journalist, space and astrobiology, deceased climber.

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