CO SELF-SHIELDING
(Isotope self-shielding = Photochemical fractionation = Isotopologue selective photodissociation)

—Some definitions from the literature—


"There are three major O-bearing species in the PMC and PPD: CO, H2O, and silicates. The primordial (i.e., molecular cloud) O-isotope compositions of these species are not known. According to the currently favored CO self-shielding model..., irradiation of gaseous CO by ultraviolet of 91–110 nm spectral range results in preferential photodissociation of minor isotopologues C17O and C18O compared to the dominant C16O that is self-shielded. The preferentially released 17O and 18O combine with hydrogen to form water that freezes out on silicate dust grains. This process results in formation of 16O-enriched CO and 17,18O-enriched H2O, but does not affect silicates. The 16O-rich and 17O-poor reservoirs with respect to solar Δ16O resulting from the CO self-shielding define the mass-independent fractionation line; however, the slope of this line is still controversial.... Note that this is not the only mechanism proposed to explain mass-independent fractionation of oxygen isotopes in the SS, but probably the most developed."

Excerpt from 'Refractory inclusions in carbonaceous chondrites: Records of early solar system processes'
A. Krot, MAPS, vol. 54, #8, (2019)
(https://doi.org/10.1111/maps.13350)

"The most widely accepted mechanism for explaining the oxygen isotopic diversity among solar system materials is CO self-shielding.... The three oxygen isotopes (16O,17O,18O) have dramatically different abundances (~2500, 1, 5, respectively). The wavelengths necessary to dissociate C16O, C17O, and C18O are distinct, and the number of photons at each wavelength is similar in the UV continuum. At the edge of a dense molecular cloud or accretion disk, UV light dissociates the same fraction of all the three isotopologues. But as the light penetrates into the cloud or disk, the photons that dissociate the C16O are depleted by absorption, so deeper in the cloud only C17O and C18O are dissociated. The resulting oxygen ions can either recombine into CO or combine with H2 to form H2O. Deeper in the cloud, the H2O will be enriched in 17O and 18O. If the solar system started out with the composition of the Sun, self-shielding would have produced isotopically heavy water in the outer parts of the disk. Yurimoto and Kuramoto (2004) also suggest that this self-shielding could occur in the pre-solar molecular cloud, and this material was transported into the solar nebula by icy dust grains during the cloud collapse. As they drifted in toward the sun and sublimated this enriched the inner disk gas."

Excerpt from 'Origin of Earth's Water: Sources and Constraints'
Karen Meech and Sean N. Raymond (chapter in forthcoming book Planetary Astrobiology, Space Science Series—February 25, 2020)
(https://arxiv.org/pdf/1912.04361.pdf)

"Self-shielding of CO in the solar nebula is a process of isotope selective photodissociation that occurs at far-ultraviolet (FUV) wavelengths from 91.2 nm to 110 nm... due to proximity to a massive O star in a star-forming region.... An FUV flux of this magnitude is expected from an O or early B star within a distance of ~1 pc of the protoSun, and implies solar birth in a cluster of ~200 stars, a very plausible birth scenario for our Solar System. During photodissociation the most abundant isotopologue (C16O) saturates, which reduces its rate of dissociation relative to the less abundant C18O and C17O. This produces a zone of enrichment of 18O and 17O, and corresponding depletion of C18O and C17O. Without concentration of H2O at the midplane, equilibration of CO and H2O as material migrated inward to ~1–2 AU would have returned both CO and H2O to the bulk isotopic values of the parent cloud. For the model presented here, concentration of ice-coated dust with Δ17O ~0 at the midplane implies grain growth timescales of ~104 years. Inward migration of ice-rich meter-sized objects may have then delivered 17O and 18O-rich water to the snowline. H2O in similarly irradiated protoplanetary disks will be enriched in 17O and 18O by ~30–100% relative to disk CO."

Excerpt from 'CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula'
J. R. Lyons and E. D. Young, Nature Letters, vol. 435 (19 May 2005)
(https://doi.org/10.1038/nature03557)

Schematic Diagram of CO Self-Shielding

Schematic diagram of CO self-shielding at the disk surface due to interstellar radiation normal to the disk and
radiation from the central star. The contours indicate decades in gas density. Saturation of C16O absorption
lines results in 17O and 18O enhancement at the disk surface (shaded region).

Diagram credit: J. R. Lyons & E. D. Young, 'Photochemical Speciation of Oxygen Isotopes in the Solar Nebula'
Published in Chondrites and the Protoplanetary Disk, ASP Conference Series, Vol. 341 (2005), A. Krot, E. Scott,
& B. Reipurth, eds.

WATER SELF-SHIELDING

"In this Letter, we implement H2O UV-shielding and chemical heating in addition to CO self-shielding within a gas-rich disk environment.... In this Letter we show that there is an enhancement in the relative H182O abundance high up in the warm molecular layer within 0.1-10 au due to self-shielding of CO, C18O, and H2O.... Much of the free 18O finds its way into water molecules, enhancing the H182O abundance, thus providing a lower ratio between H2O and H182O.... In this region, the H2O-to-H182O ratio approaches 300, a significantly lower value than the assumed ISM ratio of 550."

Excerpt from 'Water UV-shielding in the terrestrial planet-forming zone: Implications for Oxygen-18 Isotope Anomalies in H182O Infrared Emission and Meteorites'
Jenny K. Calahan, Edwin A. Bergin, and Arthur D. Bosman, Submitted to The Astrophysical Journal Letters (8 Jul 2022)
(https://arxiv.org/pdf/2207.04063v1.pdf)