Cold H2O and CO ice and gas toward the galactic center

We use the mid-IR absorption lines of gas-phase CO and H(2)O, observed with the ISO-SWS toward two sources in the Galactic Centre (Sgr A* and GCS-3), to investigate physical conditions in the cold and quiescent regions of molecular clouds along these lines of sight. Three water vapor lines of the nu (2) = 1 mode, and several lines of the R and P-branches of CO and (13)CO are detected. The analysis indicates that the absorbing gas is extremely cold, T(K) similar or equal to 10 K, confirming that in must arise in cold, quiescent regions. Column densities are similar or equal to 6.6 x 10(17) cm(-2) for CO and similar or equal to 2.0 x 10(16) cm(-2) for H(2)O toward Sgr A*, and somewhat lower values toward GCS-3. The ice phase of these molecules is also detected and quantified, and both the CO/H(2)O ratio (similar or equal to 350) and the gas/solid abundance ratio (similar or equal to 65 for CO and similar or equal to 0.02 for H(2)O) are remarkably simimlar along the two lines of sight. The fractional abundance of water vapour is similar to 3 x 10(-7), substantially lower than in the active cores of dense clouds, but much higher than recently estimated by the SWAS satellite (Snell et al. 1999). Finally, the total (gas + ice) CO/H(2)O abundance ratio is similar or equal to 5 which, assuming that all the H(2)O was originally formed in the gas phase, implies that H(2)O formation processes are highly efficient.