Detection of an absorption feature at the position of the 4.27-µm band of solid CO_2_.

Abstract

We report observations of 13 young infrared sources associated with molecular clouds with ISO's spectrophotometer ISOPHOT-S in the wavelength range from 2.5 to 11.6μm. In addition to a number of well-known interstellar bands due to various molecular ices, a new band at 4.27μm was detected, which is very likely attributed to C=O stretching vibrations of solid CO_2_. This new band is seen in all our sources. Thus, CO_2_ seems to be a common constituent of icy grain mantles in molecular clouds. The strength of the band increases with the strengths of the CO and H_2_O bands. We estimate that CO_2_ and CO may be present in comparable amounts in the mantles. The implications for the models of interstellar chemistry are briefly discussed.