Volume 572, December 2014
|Number of page(s)||6|
|Published online||04 December 2014|
Kapteyn Astronomical Institute, University of Groningen,
PO Box 800, 9700 AV
2 SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
3 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
4 Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
Accepted: 26 August 2014
Context. 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.
Aims. 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.
Methods. 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.
Results. Based on our calculations, the OPR in the Orion Bar is 0.1−0.5, which is unexpectedly low given the gas temperature of ~85 K, and also lower than the values measured for other interstellar clouds and protoplanetary disks. Toward Orion S, our OPR estimate is below 2.
Conclusions. 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.
Key words: ISM: molecules / ISM: individual objects: Orion Bar / ISM: individual objects: Orion S
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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