The water vapour abundance and its spatial distribution in Orion and Sgr B2

Jose Cernicharo ¹, E. González-Alfonso ¹, M.J. Sempere ¹, S. Leeks ², E. van Dishoeck ³, C. Wright ³, T. Lim ⁴, P. Cox ⁵, & S. Pérez-Martínez ¹

¹ Consejo Superior de Investigaciones Cientificas. Instituto de Estructura de la Materia. Dpto Fisica Molecular. C/Serrano 121. 28006 Madrid. Spain

² Queen Mary & Westfield College, London, E1 4NS, U.K.

³ Sterrewacht, Leiden, The Netherlands

⁴ ISO Science Operations Centre, Space Science Department of ESA, Villafranca, P. O. Box 50727, 28080, Madrid, Spain

⁵ IAS, Orsay, France

We present our Open Time results concerning the water vapour abundance and its spatial distribution in Orion-IRc2 and Sgr B2. LWS grating and FP data have been obtained in both sources for the main isotope of H₂O and the H₂¹⁸O and H₂¹⁷O rare isotopes. In Orion IRC2, we have observed more than 100 lines of water vapour from its different isotopes.

As already shown by Cernicharo et al. in 1994 from ground based observations at 183.3 GHz, our LWS FP and grating maps of Orion show that water vapour is spatially extended in this cloud and has an abundance 10^-5 at large scale (ridge). The same abundance has been found for the extended water vapour absorption in the direction of SgrB2 where the 179.5  H₂O line is detected over a region of 80 pc in size.

In the direction of Orion IRC2, we have detected the different gas components associated to this prominent molecular cloud. However, the interpretation of the data is not obvious in view of the extremely large opacities of the water vapour lines. We will present radiative transfer models treating simultaneously the dust and the molecules. These models predict several important effects in the water vapour lines intensities and line profiles and reproduce quantitatively the observed water vapour lines with the SWS and the LWS ISO spectrometers. A detailed fit to the data will require a more detailed knowledge of the geometry of the region and of the relative distribution of dust and H₂O. Water vapour abundances around 10^-4 have been estimated for the shocked gas in IRC2.