2020ApJ...892...22M

The depth-dependent abundance of both gas-phase and solid-state water within dense, quiescent, molecular clouds is important to both the cloud chemistry and gas cooling. Where water is in the gas phase, it is free to participate in the network of ion-neutral reactions that lead to a host of oxygen-bearing molecules, and its many ortho- and para-energy levels make it an effective coolant for gas temperatures greater than 20 K. Where water is abundant as ice on grain surfaces, and unavailable to cool the gas, significant amounts of oxygen are removed from the gas phase, suppressing the gas-phase chemical reactions that lead to a number of oxygen-bearing species, including O₂. Models of far-UV (FUV)-illuminated clouds predict that the gas-phase water abundance peaks in the range A_V ∼ 3 and 8 mag of the cloud surface, depending on the gas density and FUV field strength. Deeper within such clouds, water is predicted to exist mainly as ice on grain surfaces. More broadly, these models are used to analyze a variety of other regions, including outflow cavities associated with young stellar objects and the surface layers of protoplanetary disks. In this paper, we report the results of observational tests of FUV-illuminated cloud models toward the Orion Molecular Ridge and Cepheus B using data obtained from the Herschel Space Observatory and the Five College Radio Astronomy Observatory. Toward Orion, 2220 spatial positions were observed along the face-on Orion Ridge in the H₂O 1₁₀- 1₀₁ 557 GHz and NH₃ J, K = 1,0-0,0 572 GHz lines. Toward Cepheus B, two strip scans were made in the same lines across the edge-on ionization front. These new observations demonstrate that gas-phase water exists primarily within a few magnitudes of dense cloud surfaces, strengthening the conclusions of an earlier study based on a much smaller data set, and indirectly supports the prediction that water ice is quite abundant in dense clouds.