The abundance of water vapour in molecular clouds is one of the key problems
in the study of the interstellar medium (ISM). Although chemical models
predict high water abundance, ground-based observations of non-masering
rotational transitions of water are nearly impossible due to the terrestrial
atmospheric opacity, even from airborne platforms. Cernicharo et al (1994)
reported widespread water vapour emission in Orion at 183 GHz and derived
water abundances in excess of 10(-5), confirming that H2O is the dominant
coolant in warm molecular clouds. Recent observations using the LWS grating
have lead to spectacular results concerning the water content in the
interstellar medium. The discovery of absorption in the 179.5 micron
fundamental transition of water vapour has been reported towards galactic
compact HII regions (Cox et al. 1996) and towards the strong continuum of
Sgr B2 (Cernicharo et al. 1996). Similar results have also been found for
normal galactic lines of sights by Caux et al. (1996). The absorption likely
arises from the gas surrounding the central source and indicate the presence
of water in the associated molecular envelope and/or along the lines of sight
in the normal interstellar medium. Conservatives lower limits of the water
abundance of 10(-6) are derived from these very opaque transitions which
are readily detected with the grating spectrometer. We propose to measure
the compact HII regions for which we have evidence of water absorption by
using the LWS Fabry-Perot and the isotope H18O in order to investigate the
abundance of water more precisely. ISO is the only instrument in the next
decade which will allow to observe at high spatial resolution and sensitivity
the thermal emission of water vapor in the ISM.
These follow up observations together with similar proposals on the study of
water emission in molecular clouds could be one of the major contributions
from ISO and without doubt provide a significant progress to our undestanding
of the role of water vapour in the chemistry and physics of the ISM.