The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Understand

A very low ortho-to-para ratio (OPR) of 0.1-0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin $\mathrm{H_2^{18}O}$ lines which were analyzed by using a single-slab large velocity gradient model.

• The corresponding spin temperature does not coincide with the kinetic temperature of the molecular gas in this UV-illuminated region.
• This was interpreted as an indication of water molecules being formed on cold icy grains which were subsequently released by UV photodesorption.
• A more complete set of water observations in the Orion Bar, including seven $\mathrm{H_2^{16}O}$ lines and one $\mathrm{H_2^{18}O}$ line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR, taking into account the steep density and temperature gradients present in the region.