OH mid-infrared emission as a diagnostic of H$_2$O UV photodissociation. II. Application to interstellar PDRs

Abstract

Water photodissociation in the 114 - 144 nm UV range forms excited OH which emits at mid-infrared wavelengths via highly excited rotational lines. These lines have only been detected with Spitzer in several proto-planetary disks and shocks. Previous studies have shown they are a unique diagnostic for water photodissociation. Thanks to its high sensitivity and angular resolution, the James Webb Space Telescope (JWST) could be able to detect them in other environments such as interstellar Photo-Dissociation Regions (PDRs). In order to predict the emerging spectrum of OH, we use the Meudon PDR Code to compute the thermal and chemical structure of PDRs. The influence of thermal pressure ($P_{\rm th}/k$ = $n_{\rm H} T_{\rm K}$) and UV field strength on the integrated intensities, as well as their detectability with the JWST are studied in details. OH mid-IR emission is predicted to originate very close to the H$^0$/H$_2$ transition and is directly proportional to the column density of water photodissociated in that layer. Because neutral gas-phase formation of water requires relatively high temperatures ($T_{\rm K} \gtrsim 300~$K), the resulting OH mid-IR lines are primarily correlated with the temperature at this position, and are therefore brighter in regions with high pressure. This implies that these lines are predicted to be only detectable in strongly irradiated PDRs ($G_0^{\rm incident}$ $>$ 10$^3$) with high thermal pressure ($P_{\rm th}/k$ $\gtrsim$ 5$\times$10$^7$ K cm$^{-3}$). In the latter case, OH mid-IR lines are less dependent on the strength of the incident UV field. The detection in PDRs like the Orion bar, which should be possible, is also investigated. To conclude, OH mid-IR lines observable by JWST are a promising diagnostics for dense and strongly irradiated PDRs.