1999ApJ...513..294P

We present new 4.5-5.1 µm (2210-1970 cm^–1) spectra of embedded protostars, W33 A, AFGL 961 E, AFGL 2136, NGC 7538 IRS 9, and Mon R2 IRS 2, which contain a broad absorption feature located near 4.62 µm (2165 cm^–1), commonly referred to in the literature as the ``X-C≡N'' band. The observed peak positions and widths of the interstellar band agree to within 2.5 cm^–1 and 5 cm^–1, respectively. The strengths of the interstellar 4.62 µm band and the ice absorption features in these spectra are not correlated, which suggests a diversity of environmental conditions for the ices we are observing. We explore several possible carriers of the interstellar band and review possible production pathways through far-ultraviolet photolysis (FUV), ion bombardment of interstellar ice analog mixtures, and acid-base reactions. Good fits to the interstellar spectra are obtained with an organic residue produced through ion bombardment of nitrogen-containing ices or with the OCN^– ion produced either through acid-base reactions or FUV photolysis of NH₃-containing ices.