The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Abstract

A very low ortho-to-para ratio (OPR) of 0.1-0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin ${\mathrm{H}}_2^{18}\mathrm{O}$ lines which were analyzed by using a single-slab large velocity gradient model. The corresponding spin temperature does not coincide with the kinetic temperature of the molecular gas in this UV-illuminated region. This was interpreted as an indication of water molecules being formed on cold icy grains which were subsequently released by UV photodesorption. A more complete set of water observations in the Orion Bar, including seven ${\mathrm{H}}_2^{16}\mathrm{O}$ lines and one ${\mathrm{H}}_2^{18}\mathrm{O}$ line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR, taking into account the steep density and temperature gradients present in the region. The model line intensities are in good agreement with the observations assuming that water molecules formed with an OPR corresponding to thermal equilibrium conditions at the local kinetic temperature of the gas and when solely considering gas-phase chemistry and water gas-grain exchanges through adsorption and desorption. Gas-phase water is predicted to arise from a region deep into the cloud, corresponding to a visual extinction of $A_{\mathrm{V}}\sim 9$, with a ${\mathrm{H}}_2^{16}\mathrm{O}$ fractional abundance of $\sim 2\times 10^{-7}$ and column density of $(1.4\pm 0.8)\times 10^{15}$ cm${}^{-2}$ for a total cloud depth of $A_{\mathrm{V}}=15$. A line-of-sight average ortho-to-para ratio of $2.8\pm 0.2$ is derived. The observational data are consistent with a nuclear spin isomer repartition corresponding to the thermal equilibrium at a temperature of $36\pm 2$ K, much higher than the spin temperature previously reported for this region and close to the gas kinetic temperature in the water-emitting gas.

References in corpus (50)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• A computer program for fast non-LTE analysis of interstellar line spectra
• The distance to the Orion Nebula
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Water, O2 and Ice in Molecular Clouds
• Interstellar water chemistry: from laboratory to observations
• Photodesorption of Ices II: H2O and D2O
• Detection of the Water Reservoir in a Forming Planetary System
• Binding energies: new values and impact on the efficiency of chemical desorption
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
• Membership of the Orion Nebula Population from the Chandra Orion Ultradeep Project
• The chemistry of vibrationally excited H2 in the interstellar medium
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Observation of different reactivities of para- and ortho-water towards cold diazenylium ions
• Herschel/HIFI observations of [CII] and [13CII] in PDRs
• Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel
• Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel
• Water formation on bare grains: When the chemistry on dust impacts interstellar gas
• The CARMA-NRO Orion Survey
• H_2 Pure Rotational Lines in the Orion Bar
• Evolution of dust in the Orion Bar with Herschel: I. Radiative transfer modelling
• First detection of gas-phase ammonia in a planet-forming disk
• OH emission from warm and dense gas in the Orion Bar PDR
• X-ray emission from young brown dwarfs in the Orion Nebula Cluster
• Complex organic molecules in strongly UV-irradiated gas
• APEX 1 mm line survey of the Orion Bar
• Production of atomic hydrogen by cosmic rays in dark clouds
• Spatially extended OH+ emission from the Orion Bar and Ridge
• X-ray photodesorption from water ice in protoplanetary disks and X-ray dominated regions
• Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
• Detection of OH${}^{+}$ and H${}_2$O${}^{+}$ towards Orion~KL
• Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification
• Carbon recombination lines in the Orion Bar
• Spatially resolved images of reactive ions in the Orion Bar
• Detection of HF emission from the Orion Bar
• Oxygen fractionation in dense molecular clouds
• Constraining physical conditions for the PDR of Trumpler 14 in the Carina Nebula
• Herschel Search for O2 Toward the Orion Bar
• New measurements on water ice photodesorption and product formation under ultraviolet irradiation
• Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface
• First NH3 detection of the Orion Bar
• Spatial distribution of FIR rotationally excited CH+ and OH emission lines in the Orion Bar PDR
• Incorporation of stochastic chemistry on dust grains in the PDR code using moment equations
• A non-equilibrium ortho-to-para ratio of water in the Orion PDR
• Surface chemistry in photodissociation regions
• Confinement effects on the nuclear spin isomer conversion of H${}_2$O
• Data processing pipeline for Herschel HIFI

Cited by in corpus (13)

• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
• Probing the full CO spectral line energy distribution (SLED) in the nuclear region of a quasar-starburst system at $z=6.003$
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• Herschel observations of extraordinary sources: Full Herschel/HIFI molecular line survey of Sagittarius B2(M)
• PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. II. Application to interstellar PDRs
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• H2CO and CS in diffuse clouds: Excitation and abundance
• The nuclear-spin-forbidden rovibrational transitions of water from first principles
• H3+ in irradiated protoplanetary disks: Linking far-ultraviolet radiation and water vapor
• Weak, extended water vapor emission in the Horsehead nebula