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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 H 2 O 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, C 18 O, and H 2 O. 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 H 2 O self-shielding may effect the H 2 O 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. 

Abstract The JWST Disk Infrared Spectral Chemistry Survey (JDISCS) aims to understand the evolution of the chemistry of inner protoplanetary disks using the Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST). With a growing sample of >30 disks, the survey implements a custom method to calibrate the MIRI Medium Resolution Spectrometer (MRS) to contrasts of better than 1:300 across its 4.9–28μm spectral range. This is achieved using observations of Themis family asteroids as precise empirical reference sources. The high spectral contrast enables precise retrievals of physical parameters, searches for rare molecular species and isotopologues, and constraints on the inventories of carbon- and nitrogen-bearing species. JDISCS also offers significant improvements to the MRS wavelength and resolving power calibration. We describe the JDISCS calibrated data and demonstrate their quality using observations of the disk around the solar-mass young star FZ Tau. The FZ Tau MIRI spectrum is dominated by strong emission from warm water vapor. We show that the water and CO line emission originates from the disk surface and traces a range of gas temperatures of ∼500–1500 K. We retrieve parameters for the observed CO and H₂O lines and show that they are consistent with a radial distribution represented by two temperature components. A high water abundance ofn(H₂O) ∼ 10⁻⁴fills the disk surface at least out to the 350 K isotherm at 1.5 au. We search the FZ Tau environs for extended emission, detecting a large (radius of ∼300 au) ring of emission from H₂gas surrounding FZ Tau, and discuss its origin. 

Abstract Mid-infrared spectroscopy is one of the few ways to observe the composition of the terrestrial planet-forming zone, the inner few astronomical units, of protoplanetary disks. The species currently detected in the disk atmosphere, for example, CO, CO₂, H₂O, and C₂H₂, are theoretically enough to constrain the C/O ratio on the disk surface. However, thermochemical models have difficulties in reproducing the full array of detected species in the mid-infrared simultaneously. In an effort to get closer to the observed spectra, we have included water UV-shielding as well as more efficient chemical heating into the thermochemical code Dust and Lines. We find that both are required to match the observed emission spectrum. Efficient chemical heating, in addition to traditional heating from UV photons, is necessary to elevate the temperature of the water-emitting layer to match the observed excitation temperature of water. We find that water UV-shielding stops UV photons from reaching deep into the disk, cooling down the lower layers with a higher column. These two effects create a hot emitting layer of water with a column of 1–10 × 10¹⁸cm⁻². This is only 1%–10% of the water column above the dustτ= 1 surface at mid-infrared wavelengths in the models and represents <1% of the total water column. 

Abstract Carbon dioxide is an important tracer of the chemistry and physics in the terrestrial planet-forming zone. Using a thermochemical model that has been tested against the mid-infrared water emission, we reinterpret the CO₂emission as observed with Spitzer. We find that both water UV-shielding and extra chemical heating significantly reduce the total CO₂column in the emitting layer. Water UV-shielding is the more efficient effect, reducing the CO₂column by ∼2 orders of magnitude. These lower CO₂abundances lead to CO₂-to-H₂O flux ratios that are closer to the observed values, but CO₂emission is still too bright, especially in relative terms. Invoking the depletion of elemental oxygen outside of the water midplane ice line more strongly impacts the CO₂emission than it does the H₂O emission, bringing the CO₂-to-H₂O emission in line with the observed values. We conclude that the CO₂emission observed with Spitzer-IRS is coming from a thin layer in the photosphere of the disk, similar to the strong water lines. Below this layer, we expect CO₂not to be present except when replenished by a physical process. This would be visible in the¹³CO₂spectrum as well as certain¹²CO₂features that can be observed by JWST-MIRI. 

Abstract Theoretical models and observations suggest that the abundances of molecular ions in protoplanetary disks should be highly sensitive to the variable ionization conditions set by the young central star. We present a search for temporal flux variability of HCO⁺J= 1–0, which was observed as a part of the Molecules with Atacama Large Millimeter/submillimeter Array (ALMA) at Planet-forming Scales ALMA Large Program. We split out and imaged the line and continuum data for each individual day the five sources were observed (HD 163296, AS 209, GM Aur, MWC 480, and IM Lup, with between three and six unique visits per source). Significant enhancement (>3σ) was not observed, but we find variations in the spectral profiles in all five disks. Variations in AS 209, GM Aur, and HD 163296 are tentatively attributed to variations in HCO⁺flux, while variations in IM Lup and MWC 480 are most likely introduced by differences in theuvcoverage, which impact the amount of recovered flux during imaging. The tentative detections and low degree of variability are consistent with expectations of X-ray flare-driven HCO⁺variability, which requires relatively large flares to enhance the HCO⁺rotational emission at significant (>20%) levels. These findings also demonstrate the need for dedicated monitoring campaigns with high signal-to-noise ratios to fully characterize X-ray flare-driven chemistry.