The interpretation of water emission from dense interstellar clouds

¹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: D.R.Poelman@astro.rug.nl
² SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
³ NASA Ames Research Center, MS245-3, Moffett Field, CA 94035, USA

Received: 4 August 2006
Accepted: 11 January 2007

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- () 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(, the intensity of the transition scales with the total column density of . However, this relationship breaks down with increasing abundance, i.e., optical depth, due to line trapping and – for  K, X( and – absorption of the dust continuum.

Conclusions.An observed decline in intensity per column density, expected if 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.