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: Investigate wether the ground-state transition of ortho-H2O, 1_(10)-1_(01), at 557GHz 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 the treatment of molecular line emission. Results: For low abundances, i.e., X(H2O)=<10^(-9), the intensity of the 1_(10)-1_(01) transition scales with the total column 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~10^4 cm^(-3) -- absorption of the dust continuum. Conclusions: An observed decline in intensity per column, expected if H2O is a surface tracer, does not necessarily mean that the water is absent in the gas phase at large columns, but can be caused by line trapping and subsequent collisional de-excitation. To determine the amount of water vapour in the interstellar medium, multiple line measurements of optically thin transitions are needed to disentangle radiative transfer and local excitation effects.

5 pages, 5 figures

References in corpus (6)

• Water Abundance in Molecular Cloud Cores
• CN and HCN in Dense Interstellar Clouds
• Submillimeter Wave Astronomy Satellite Observations of Extended Water Emission in Orion
• The excitation of water in the S140 photon dominated region
• Water Absorption From Line-of-Sight Clouds Toward W49A
• Line profiles of water for the photon dominated region and embedded sources in the S140 region

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