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 × 1018 cm−2. This is only 1%–10% of the water column above the dust τ = 1 surface at mid-infrared wavelengths in the models and represents <1% of the total water column.

Water UV-shielding in the Terrestrial Planet-forming Zone: Implications for Oxygen-18 Isotope Anomalies in H218O Infrared Emission and Meteorites

“…In order to determine which water lines will be of high interest, we run a quick non-LTE ray-tracing program (Bosman et al 2017, Appendix B) that estimates the flux of water lines over the full extent of the MIRI wavelength range. We find that IR bright H 2 O lines emit high up in the disk, proving to be optically thick across all IR wavelengths (see A. D. Bosman et al 2022, in using the relative spatial contribution from where each line is emitting from, as calculated by DALI.…”

“…These two processes are required in order for a thermochemical calculation to reproduce water line emission and observed excitation temperatures from Spitzer. Using the same modeling setup, Bosman et al (2022) show that CO 2 -to-H 2 O ratios can also be achieved, strengthened by additional depletion of excess oxygen beyond the water snowline. S. Duval et al (2022, in preparation) will focus on the impact of water UV-shielding on the chemical composition of the terrestrial planet-forming region.…”

“…It contains an isotope chemical network (Miotello et al 2014) and accounts for molecular selfshielding. For this project, H 2 O UV-shielding has been additionally accounted for; see Bosman et al (2022) for details. We utilize a model derived from the AS 209 and corresponding stellar spectrum from Zhang et al (2021), and explore the effect of H 2 O UV-shielding and Lyα radiation in a flat versus thick model and two levels of dust settling.…”

“…Due to vertical mixing, water from the gaseous atmosphere can transport down to the midplane where it locked onto grains as ice. The cold finger effect has been used to reconcile models in which high water abundances are predicted, yet corresponding submillimeter observations exhibit nondetections (Salyk et al 2011;Du et al 2015;Carr et al 2018;Bosman & Bergin 2021;Bosman et al 2022). Thus, while the H O 2 18 -rich regions reside high up in the disk atmosphere, vertical mixing can bring a portion of this reservoir down to the midplane and enrich meteoritic precursors.…”

“…Duval et al (2022, in preparation). Bosman et al (2022) set up protoplanetary disk models focused on the innermost region that take into account water UV-shielding and efficient chemical heating processes. These two processes are required in order for a thermochemical calculation to reproduce water line emission and observed excitation temperatures from Spitzer.…”