2009ApJ...690.1497H

We model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV; 6 eV <hν < 13.6 eV) flux G₀(scaling factor in multiples of the local interstellar field). Extending previous photodissociation region (PDR) models, we include the freezing of species, simple grain surface chemistry, and desorption (including FUV photodesorption) of ices. We also treat the opaque cloud interior with time-dependent chemistry. Here, under certain conditions, gas-phase elemental oxygen freezes out as water ice and the elemental C/O abundance ratio can exceed unity, leading to complex carbon chemistry. Gas-phase H₂ O and O₂ peak in abundance at intermediate depth into the cloud, roughly A_V∼ 3-8 from the surface, the depth proportional to ln(G₀/n). Closer to the surface, molecules are photodissociated. Deeper into the cloud, molecules freeze to grain surfaces. At intermediate depths, PDRs are attenuated by dust extinction, but photodesorption prevents total freeze-out. For G₀< 500, abundances of H₂ O and O₂peak at values ∼10^–7, producing columns ∼10¹⁵/cm2, independent of G₀ and n. The peak abundances depend primarily on the product of the photodesorption yield of water ice and the grain surface area per H nucleus. At higher values of G₀, thermal desorption of O atoms from grains slightly enhances the gas-phase H₂ O peak abundance and column, whereas the gas-phase O₂ peak abundance rises to ∼10^–5 and the column to ∼2 x 10¹⁶/cm2. We present simple analytical equations for the abundances as a function of depth, which clarify the dependence on parameters. The models are applied to observations of H₂O, O₂, and water ice in a number of sources, including B68, NGC 2024, and ρ Oph.