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 ($P_{\mathrm{th}}/k$ = $n_{\mathrm{H}}{T}_{\mathrm{K}}$) 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 ($T_{\mathrm{K}}\gtrsim 300$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 ($G_0^{\mathrm{incident}}$ $>$ 10${}^3$) with high thermal pressure ($P_{\mathrm{th}}/k$ $\gtrsim$ 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.

Accepted on the 17/10/2022 by Astronomy & Astrophysics (A&A)

References in corpus (19)

• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Photodissociation and X-Ray Dominated Regions
• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• Expanding bubbles in Orion A: [CII] observations of M42, M43, and NGC 1977
• Discovery of superthermal hydroxyl (OH) in the HH211 outflow
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. I. Model and application to the HH 211 shock
• SOC program for dust continuum radiative transfer
• Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface
• Origin of warm and hot gas emission from low-mass protostars: Herschel-HIFI observations of CO J=16-15. I. Line profiles, physical conditions, and H2O abundance
• A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions
• Spatial distribution of FIR rotationally excited CH+ and OH emission lines in the Orion Bar PDR
• A non-equilibrium ortho-to-para ratio of water in the Orion PDR
• Dust evolution across the Horsehead Nebula
• The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
• Quantum study of reaction O(3P) + H2(v,j) $\to$ OH + H: OH formation in strongly UV-irradiated gas
• Factories of CO-dark gas: molecular clouds with limited star formation efficiencies by FUV feedback
• Nano-grain depletion in photon-dominated regions

Cited by in corpus (6)

• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. III. Application to planet-forming disks
• PDRs4All XI. Detection of infrared CH${}^{+}$ and CH${}_3^{+}$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
• XUE. JWST spectroscopy of externally irradiated disks around young intermediate-mass stars
• Improving Accretion Diagnostics for Young Stellar Objects with Mid-infrared Hydrogen lines from JWST/MIRI