2014A&A...572L..10C

The ortho-to-para ratio (OPR) of H₂O is thought to be sensitive to the temperature of water formation. The OPR of H₂O is thus useful for studying the formation mechanism of water. We investigate the OPR of water in the Orion PDR (photon-dominated region), at the Orion Bar and Orion S positions, using data from Herschel/HIFI. We detect the ground-state lines of ortho- and para-H₂¹⁸O in the Orion Bar and Orion S and estimate the column densities using local thermodynamical equilibrium (LTE) and non-LTE methods. Based on our calculations, the OPR in the Orion Bar is 0.1-0.5, which is unexpectedly low given the gas temperature of ∼85K, and also lower than the values measured for other interstellar clouds and protoplanetary disks. Toward Orion S, our OPR estimate is below 2. This low OPR at 2 positions in the Orion PDR is inconsistent with gas phase formation and with thermal evaporation from dust grains, but it may be explained by photodesorption.