Cold H2O and CO ice and gas toward the Galactic center

We present ice- and gas-phase observations of CO, (CO)-C-13 and of H2O in the mid- and far-infrared taken with the Infrared Space Observatory (ISO) short- and long-wavelength spectrometers toward two positions in the Galactic center region (Sagittarius A* and GCS 3). These data are complemented with Caltech Submillimeter Observatory (CSO) observations of the J = 3-2 and J = 7-6 lines of CO. The ISO and CSO data indicate that the absorbing gas is extremely cold, T-K similar or equal to 10 K, suggesting that it is located in the dark clouds of the different spiral arms that intersect the lines of sight. From the analysis of the CO absorption, we derive (CO)-C-13 gas-phase column densities of 1.1 and 0.7 x 10(17) cm(-2) toward Sgr A* and GCS 3, respectively. The H2O gas column density in the direction of Sgr A* is similar or equal to 2 x 10(16) cm(-2). The derived CO/H2O and gas/solid abundance ratios corresponding to these cold clouds are remarkably similar along the two lines of sight. We find that nearly all the CO is in the gas phase, while the H2O is almost entirely frozen onto dust grains. Finally, the Ngas+ice(CO)/Ngas+ice(H2O) abundance ratio is similar or equal to5, implying that H2O formation processes are highly efficient.