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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.

 

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. 

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.