Abstract
Context. The ortho-to-para ratio (OPR) of water in the interstellar medium (ISM) is often assumed to be related to the formation temperature of water molecules, making it a potentially interesting tracer of the thermal history of interstellar gas. Aims. A very low OPR of 0.1–0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin H218O lines which were analyzed by using a single-slab large velocity gradient (LVG) 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. Methods. A more complete set of water observations in the Orion Bar, including seven H216O lines and one H218O line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR. The model takes into account the steep density and temperature gradients present in the region. Results. 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 AV ~ 9, with a H216O fractional abundance of ~2 × 10−7 and column density of (1.4 ± 0.8) × 1015 cm−2 for a total cloud depth of AV = 15. A line-of-sight average OPR of 2.8 ± 0.2 is derived. Conclusions. The observational data are consistent with a nuclear spin isomer repartition corresponding to the thermal equilibrium at a temperature of 36 ± 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.
Highlights
Photon-dominated regions (PDRs) are surface layers of molecular clouds irradiated by a strong UV photon flux, such as those present in star-forming regions
The Orion Molecular Cloud 1 (OMC-1) contains two embedded star-forming regions: Orion BN/KL and Orion South, as well as a group of young massive stars known as the Trapezium Cluster
Self-absorption The analysis of water line intensities with the Meudon PDR code brings a justification for the assumption made to derive the observed intensities
Summary
Photon-dominated regions (PDRs) are surface layers of molecular clouds irradiated by a strong UV photon flux, such as those present in star-forming regions. The Trapezium, and in particular the brightest O6 type star Θ1 Ori C, has irradiated the surrounding molecular cloud creating an HII region, which is bordered by the Orion Bar on its southeastern side and by the Orion Ridge on its western side (Rodriguez-Franco et al 1998). Close to Earth (414 pc; Menten et al 2007) and with a nearly edge-on geometry (Hogerheijde et al 1995; Jansen et al 1995) explained by the blister model (see for example Wen & O’Dell 1995 and references therein), the Orion Bar is an excellent place to test PDR models
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