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Astron. Astrophys. 323, L25-L28 (1997)

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Letter to the Editor

Widespread water vapour absorption in SgrB21

J. Cernicharo 1, T. Lim 2, P. Cox 3, E. González-Alfonso 4, 5, E. Caux 6, B.M. Swinyard 7, J. Martín-Pintado 5, J.P. Baluteau 8 and  P. Clegg9

1 CSIC, IEM, Dpto. Física Molecular, Serrano 123, E-28006 Madrid, Spain
2 The LWS Instrument-Dedicated Team, ISO Science Operations Centre. PO Box 50727, E-28080 Madrid, Spain
3 Institut d'Astrophysique Spatiale, Bât. 121, Université de Paris XI, F-91405 Orsay Cedex, France
4 Universidad de Alcalá de Henares, Dpto. Física, Campus Universitario, E-28871 Alcalá de Henares, Spain
5 Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain
6 Centre d'Etude Spatiale des Rayonnements. CESR/CNRS-UPS, BP 4346, F-31029 Toulouse Cedex. France
7 Rutherford Appleton Lab., Chilton, Didcot, Oxon OX11 OQX. UK
8 Laboratoire d'Astronomie Spatiale, LP CNRS, BP 8, F-13376 Marseille Cedex 12, France
9 Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK

Received 28 November 1996 / Accepted 29 April 1997


We report the discovery of widespread water vapour absorption in the 212 -101 179.5 µm transition toward the SgrB2 molecular cloud. The data were obtained with the Long Wavelength Spectrometer (LWS) on board the ISO satellite. A raster map centered on SgrB2 made with the LWS grating clearly shows the 179.5 µm H2 O line in absorption against the far-infrared continuum at all observed positions with a line absorption depth of [FORMULA]  15%. The grating data are complemented with Fabry-Perot observations of the H2 O 179.5 and 174.6  [FORMULA] lines, and the H [FORMULA] O 181.05  [FORMULA]. The transitions connecting the ground level show a broad absorption profile between -150 and 100 kms-1 proving that water vapour is present in the molecular gas along the line of sight toward SgrB2. The H2 O absorption lines probably arise in the tenuous and extended envelope of SgrB2 where collisional excitation is negligible and the excitation is mainly due to absorption of photons emitted by the dust. We derive a lower limit to the H2 O abundance in SgrB2 of 10-5.

Key words: Line: Identification – Molecular Processes – ISM: individual objects: Sgr B2 – ISM: molecules

Send offprint requests to: J. Cernicharo

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998