Cold H2O and CO ice and gas toward the Galactic Center
Abstract
We use the mid-IR absorption lines of gas-phase CO and h2o, 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 ν2=1 mode, and several lines of the R and P-branches of CO and 13CO are detected. The analysis indicates that the absorbing gas is extremely cold, TK ≃ 10 K, confirming that in must arise in cold, quiescent regions. Column densities are ≃ 6.6 × 1017sci for CO and ≃ 2.0 × 1016sci for h2o\ toward sgra, and somewhat lower values toward GCS-3. The ice phase of these molecules is also detected and quantified, and both the CO/h2o\ ratio (≃ 350) and the gas/solid abundance ratio (≃ 65 for CO and ≃ 0.02 for h2o) are remarkably simimlar along the two lines of sight. The fractional abundance of water vapour is ~ 3 × 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/H2O abundance ratio is ≃ 5 which, assuming that all the H2O was originally formed in the gas phase, implies that H2O formation processes are highly efficient.
- Publication:
-
ISO Beyond the Peaks: The 2nd ISO Workshop on Analytical Spectroscopy
- Pub Date:
- November 2000
- Bibcode:
- 2000ESASP.456..119M
- Keywords:
-
- ISM: MOLECULES;
- LINE: PROFILES;
- INFRARED