4.6 micron absorption features due to solid phase CO and cyano group molecules toward compact infrared sources

The authors have obtained 4.55 - 4.73 μm spectra of W33A and six other compact molecular cloud sources with a resolving power of 840, in order to resolve the absorption feature at 4.61 μm. The 4.61 μm band splits into three features which vary in relative strength from source to source. The source spectra are compared with laboratory spectra obtained specifically to study the spectral signatures of simple molecular "ices" whose composition is chosen to mimic interstellar grain mantles. These comparisons strongly support the identification with solid CO, demonstrate the presence of the cyano group, C≡N, and show that the variations in relative band strengths can be explained in terms of variations in the composition, the amount of ultraviolet irradiation, and the temperature of the material. The authors discuss the astrophysical conditions under which the molecules responsible for the observed features are present in grain mantles.