A search for interstellar gas-phase CO_2_. Gas: solid state abundance ratios.
de Graauw, T.; van Dishoeck, E. F.; Salama, A.; Whittet, D. C. B.; Schutte, W. A.; Tielens, A. G. G. M.; Boogert, A. C. A.; Ehrenfreund, P.; Gerakines, P. A.; Helmich, F. P.; Black, J. H.; Roelfsema, P. R.; Vandenbussche, B.; Lacy, J. H.; Boxhoorn, D. R.; Kester, D. J. M.; Leech, K.; Millar, T. J.
Abstract
We present searches for gas-phase CO_2_ features in the ISO-SWS infrared spectra of four deeply embedded massive young stars, which all show strong solid CO_2_ absorption. The abundance of gas-phase CO_2_ is at most 2x10^-7^ with respect to H_2_, and is less than 5% of that in the solid phase. This is in strong contrast to CO, which is a factor of 10-100 more abundant in the gas than in solid form in these objects. The gas/solid state ratios of CO_2_, CO and H_2_O are discussed in terms of the physical and chemical state of the clouds.