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

Article (PDF Available)inThe Astrophysical Journal 570:198-209 · May 2002with24 Reads
DOI: 10.1086/339570
Abstract
We discuss 2.8-3.9 mum 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 mum 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 pro le is not fully accounted for by pure H2O ice mantles. Residual absorption is present at 2.95 and 3.2-3.6 l m. Aliphatic hydrocarbon absorption at 3.4 mum 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 pro le for the GC line of sight, we reveal an additional broad absorption component around similar to3.3 mum, which partially underlies the similar to3.4 mum aliphatic hydrocarbon feature. Its carrier resides in the diffuse interstellar medium. The width of this absorption is deduced to be at least similar to100 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 mum "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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Available from: Yvonne Pendleton, Dec 15, 2013