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

Context. The Mid-InfraRed Instrument (MIRI) on board the James Webb Space Telescope (JWST) gives unique access to the physical and chemical structure of inner disks (<10 au), where the majority of the planets are forming. However, the interpretation of mid-infrared (mid-IR) spectra requires detailed thermo-chemical models able to provide synthetic spectra readily comparable to spectroscopic observations. This is particularly important for OH, which can be excited by a number of processes. Aims. Our goal is to explore the potential of mid-IR emission of OH to probe H2O photodissociation, and thus implicitly the far-ultraviolet (FUV) radiation field in the inner disks. Methods. We include in the DALI disk model prompt emission of OH following photodissociation of H2O in its B electronic state by photons at lambda < 144 nm. Compared with previous modeling work, we also take into account the propensity of forming OH in the A ' symmetric states. This model allows us to compute in a self-consistent manner the thermal and chemical structure of the disk and the resulting mid-IR line intensities of OH and H2O. Results. The OH line intensities in the 9-13 mu m range are proportional to the total amount of water photodissociated in the disk. As such, these OH lines are a sensitive tracer of the amount of H2O exposed to the FUV field, which depends on the temperature, density, and strength of the FUV field reaching the upper molecular layers. In particular, we show that the OH line fluxes primarily scale with the FUV field emitted by the central star in contrast with H2O lines in the 10-20 mu m range which scale with the bolometric luminosity. OH is therefore an important diagnostic to probe the effect of Ly alpha and constrain the dust FUV opacity in upper molecular layers. A strong asymmetry between the A ' and A '' components of each rotational quadruplet is predicted. Conclusions. OH mid-IR emission is a powerful tool to probe H2O photodissociation and infer the physical conditions in disk atmospheres. As such, the inclusion of OH mid-IR lines in the analysis of JWST-MIRI spectra will be crucial for robustly inferring the chemical composition of planet-forming disks. The interpretation of less excited OH lines in the MIRI-MRS range requires additional quantum calculations of the formation pumping of OH (ro-)vibrational levels by O+H-2 and the collisional rate coefficients.