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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.
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Water UV-shielding in the Terrestrial Planet-forming Zone: Implications from Water Emission
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.
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- Award ID(s):
- 1907653
- PAR ID:
- 10367330
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 930
- Issue:
- 2
- ISSN:
- 2041-8205
- Format(s):
- Medium: X Size: Article No. L26
- Size(s):
- Article No. L26
- Sponsoring Org:
- National Science Foundation
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