The interpretation of water emission from dense interstellar clouds

Abstract

Context. Existing SWAS observations and future HIFI/Herschel data require a clear sense of the information content of water emission and absorption lines. Aims. We investigate whether the ground-state transition of ortho-H2O (110 → 101) at 557 GHz can be used to measure the column density throughout an interstellar cloud. Methods. We make use of a multi-zone escape probability code suitable for treating molecular line emission. Results. 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 Tdust ≳25 K, X(H2O) ≲ 10-8 and n ∼ 104 cm-3 - absorption of the dust continuum. Conclusions. 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 deexcitation. 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. © ESO 2007.