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

TL;DR

This study measures the water ortho-to-para ratio in the Orion PDR, revealing unexpectedly low values that challenge existing formation theories and suggest photodesorption effects.

Contribution

It provides the first detailed measurements of water OPR in the Orion PDR, highlighting deviations from expected thermal equilibrium values.

Findings

OPR in Orion Bar is 0.1-0.5, lower than expected for 85 K gas.

OPR toward Orion S is below 2, indicating non-thermal formation processes.

Results suggest photodesorption influences water OPR in the region.

Abstract

The ortho-to-para ratio (OPR) of H$_2$O is thought to be sensitive to the temperature of water formation. The OPR of H$_2$O is thus useful to study 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 {\it Herschel}/HIFI. We detect the ground-state lines of ortho- and para-H$_2$$^{18}$O in the Orion Bar and Orion S and we estimate the column densities using LTE and non-LTE methods. Based on our calculations, the ortho-to-para ratio (OPR) in the Orion Bar is 0.1 $-$ 0.5, which is unexpectedly low given the gas temperature of $\sim$ 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. 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.