Water UV-shielding in the Terrestrial Planet-forming Zone: Implications for Carbon Dioxide Emission
Arthur D. Bosman, Edwin A. Bergin, Jenny K. Calahan, Sara E. Duval
Abstract
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 CO 2 emission as observed with Spitzer. We find that both water UV-shielding and extra chemical heating significantly reduce the total CO 2 column in the emitting layer. Water UV-shielding is the more efficient effect, reducing the CO 2 column by ∼2 orders of magnitude. These lower CO 2 abundances lead to CO 2 -to-H 2 O flux ratios that are closer to the observed values, but CO 2 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 CO 2 emission than it does the H 2 O emission, bringing the CO 2 -to-H 2 O emission in line with the observed values. We conclude that the CO 2 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 CO 2 not to be present except when replenished by a physical process. This would be visible in the 13 CO 2 spectrum as well as certain 12 CO 2 features that can be observed by JWST-MIRI.