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