2022ApJ...933L..40B

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 CO₂ emission as observed with Spitzer. We find that both water UV-shielding and extra chemical heating significantly reduce the total CO₂ column in the emitting layer. Water UV-shielding is the more efficient effect, reducing the CO₂ column by ∼2 orders of magnitude. These lower CO₂ abundances lead to CO₂-to-H₂O flux ratios that are closer to the observed values, but CO₂ emission 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 CO₂ emission than it does the H₂O emission, bringing the CO₂-to-H₂O emission in line with the observed values. We conclude that the CO₂ emission 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 CO₂ not to be present except when replenished by a physical process. This would be visible in the ¹³CO₂ spectrum as well as certain ¹²CO₂ features that can be observed by JWST-MIRI.