The interpretation of water emission from dense interstellar clouds

Free access article

Issue		A&A Volume 464, Number 3, March IV 2007


Page(s)		1023 - 1027
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:20066184

A&A 464, 1023-1027 (2007)
DOI: 10.1051/0004-6361:20066184

The interpretation of water emission from dense interstellar clouds

D. R. Poelman^{1, 2}, M. Spaans¹, and A. G. G. M. Tielens^{3, 1}

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- H₂O ( $\mathrm{1_{10}}\rightarrow\mathrm{1_{01}}$ ) 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( $\mathrm{H_2O}) \lesssim 10^{-9}$ , the intensity of the $\mathrm{1_{10}}\rightarrow\mathrm{1_{01}}$ 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 $T\mathrm{_{dust}}\gtrsim25$ K, X( $\mathrm{H_2O})\lesssim10^{-8}$ and $n\sim10^4~\mathrm{cm^{-3}}$ - 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 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.

Key words: ISM: molecules -- radiative transfer -- ISM: dust, extinction

© ESO 2007

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