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 dust
The chemical composition of the inner region of protoplanetary disks can trace the composition of planetary-building material. The exact elemental composition of the inner disk has not yet been measured and tensions between models and observations still exist. Recent advancements have shown UV shielding to be able to increase the emission of organics. Here, we expand on these models and investigate how UV shielding may impact chemical composition in the inner 5 au. In this work, we use the model from Bosman et al. and expand it with a larger chemical network. We focus on the chemical abundances in the upper disk atmosphere where the effects of water UV shielding are most prominent and molecular lines originate. We find rich carbon and nitrogen chemistry with enhanced abundances of C2H2, CH4, HCN, CH3CN, and NH3by >3 orders of magnitude. This is caused by the self-shielding of H2O, which locks oxygen in water. This subsequently results in a suppression of oxygen-containing species like CO and CO2. The increase in C2H2seen in the model with the inclusion of water UV shielding allows us to explain the observed C2H2abundance without resorting to elevated C/O ratios as water UV shielding induced an effectively oxygen-poor environment in oxygen-rich gas. Thus, water UV shielding is important for reproducing the observed abundances of hydrocarbons and nitriles. From our model result, species like CH4, NH3, and NO are expected to be observable with the James Webb Space Telescope (JWST).
more » « less- Award ID(s):
- 1907653
- NSF-PAR ID:
- 10484959
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 934
- Issue:
- 2
- ISSN:
- 2041-8205
- Format(s):
- Medium: X Size: Article No. L25
- Size(s):
- ["Article No. L25"]
- Sponsoring Org:
- National Science Foundation
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