Cold H[TINF]2[/TINF]O and CO Ice and Gas toward the Galactic Center

We present observations of CO, 13CO and of H2O in the middle and far-infraredtaken with the ISO-SWS and ISO-LWS spectrometers toward two positions in theGalactic Center region (Sgr A* and GCS-3). Both ice and gas phase molecules aredetected. The ISO data have been complemented with observations of the J=3-2and J=7-6 lines of CO carried out at the Caltech Submillimeter Observatory. TheISO 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 armsthat intersect the line of sight towards the Galactic Center. From the analysisof the CO absorption we derive 13CO gas phase column densities of 1.1 and0.7E17 cm-2 towards Sgr A* and GCS-3, respectively. The H2O gas column densityin the direction of Sgr A* is ~ 2E16 cm-2. The derived CO/H2O and gas/solidabundance ratios corresponding to these cold clouds are remarkably similaralong the two lines of sight. We find that nearly all the CO is in the gasphase, while the H2O is almost entirely frozen onto the surfaces of cold dustgrains. Finally, the N_{gas+ice}(CO)/N_{gas+ice}(H2O) abundance ratio is ~5implying that H2O formation processes are highly efficient.Comment: Accepted by ApJ Letter