2001ApJ...549L.203M

We present ice- and gas-phase observations of CO, ¹³CO 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≃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 ¹³CO gas-phase column densities of 1.1 and 0.7x10¹⁷ cm^–2 toward Sgr A* and GCS 3, respectively. The H₂O gas column density in the direction of Sgr A* is ≃2x10¹⁶ 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 N_gas+ice(CO)/N_gas+ice(H₂O) abundance ratio is ≃5, implying that H₂O formation processes are highly efficient.