A search for interstellar gas--phase CO
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 2: 10 Gamma7 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. Key words: ISM: abundances; ISM: molecules; ISM: individual: AFGL 2591, AFGL 4176, AFGL 2136, NGC 7538 IRS9 1.