Carbon Dioxide is an important tracer of the chemistry and physics in the
terrestrial planet forming zone. Using a thermo-chemical model that has been
tested against the mid-infrared water emission we re-interpret 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 mid-plane iceline 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 photo-sphere 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.Comment: 8 pages, 4 figures, accepted for publication in ApJ