2007A&A...464.1023P

Existing SWAS observations and future HIFI/Herschel data require a clear sense of the information content of water emission and absorption lines. We investigate whether the ground-state transition of ortho-H₂O (1₁₀->1₀₁) at 557GHz can be used to measure the column density throughout an interstellar cloud. We make use of a multi-zone escape probability code suitable for treating molecular line emission. For low abundances, i.e., X(H₂O)≲10^–9, the intensity of the 1₁₀->1₀₁ transition scales with the total column density of H₂. However, this relationship breaks down with increasing abundance, i.e., optical depth, due to line trapping and - for T_dust>25K, X(H₂O)≲10^–8 and n∼10⁴cm^–3 - absorption of the dust continuum. An observed decline in intensity per column density, expected if H₂O is a surface tracer, does not necessarily mean that the water is absent in the gas phase at large column densities, but can be caused by line trapping and subsequent collisional de-excitation. To determine the amount of water vapor in the interstellar medium, multiple line measurements of optically thin transitions are needed to disentangle radiative transfer and local excitation effects.