Hydrocarbons, Ices, and “XCN” in the Line of Sight toward the Galactic Center

We discuss 2.8-3.9 μm spectra from the United Kingdom Infrared Telescope of seven sight lines toward IR sources near Sagittarius A* in the Galactic center (GC). In all lines of sight, the 3.0 μm H2O ice feature is present with optical depths in the range 0.33-1.52. By constructing a simple ice model, we show that the ice profile is not fully accounted for by pure H2O ice mantles. Residual absorption is present at 2.95 and 3.2-3.6 μm. Aliphatic hydrocarbon absorption at 3.4 μm is shown to vary by a factor of 1.7, indicating significant changes in the foreground extinction across the small field. By determining the true ice profile for the GC line of sight, we reveal an additional broad absorption component around ~3.3 μm, which partially underlies the 3.4 μm aliphatic hydrocarbon feature. Its carrier resides in the diffuse interstellar medium. The width of this absorption is deduced to be at least ~100 cm-1, much broader than individual polycyclic aromatic hydrocarbon molecules produced in the laboratory or unidentified infrared emission features observed in the interstellar medium. The 4.62 μm "XCN" feature is detected in the molecular clouds along the line of sight toward IRS 19. In the solar neighborhood, this feature is seen only toward some deeply embedded protostars. Toward the GC, it may indicate the serendipitous presence of such an object in the line of sight to IRS 19, or it might conceivably arise from the processing of ices in the circumnuclear ring of the GC itself.

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