A non-equilibrium ortho-to-para ratio of water in the Orion PDR^⋆,⋆⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: y.choi@astro.rug.nl
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

Received: 16 April 2014
Accepted: 26 August 2014

Abstract

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.