OH mid-infrared emission as a diagnostic of H2O UV photodissociation

Abstract

Context. Water photodissociation in the 114–143 nm UV range forms excited OH which emits at mid-infrared (MIR) wavelengths via highly excited rotational lines. These lines have only been detected with Spitzer in proto-planetary disks and shocks. Previous studies have shown that 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 photodissociation regions (PDRs). Aims. Our goal is to predict OH MIR lines for a large range of thermal pressures and UV fields in PDRs. Methods. We use the Meudon PDR Code to compute the thermal and chemical structure of PDRs. In order to predict the emerging spectrum of OH, we amended the code to include prompt emission induced by H2O photodissociation between 114 and 143 nm. We performed a detailed study of the influence of thermal pressure (Pth/k = nHTK) and UV field strength on the integrated intensities and their detectability with the JWST. Results. OH MIR emission is predicted to originate very close to the H0/H2 transition and is directly proportional to the column density of water photodissociated in that layer. Because gas-phase neutral-neutral reactions forming water require relatively high temperatures (TK ≳ 300 K), the resulting OH MIR 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 (G0incident > 103) with high thermal pressure (Pth/k ≳ 5x107 K cm–3). In the latter case, OH MIR lines are less dependent on the strength of the incident UV field. The detection of such lines in PDRs such as the Orion bar – which should be possible – is also investigated and we show that the line-to-continuum ratio could be a major limitation for detection because of instrumental limitations. Conclusions. OH MIR lines observable by JWST are a promising diagnostic for dense and strongly irradiated PDRs and proplyds. Their intensities are directly proportional to the amount of water photodissociated and they are therefore an indirect but sensitive probe of the gas temperature at the H0/H2 transition.