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Radiative Trapping and Hyperfine Structure: HCN (11),
OWENS VALLEY RADIO OBSERVATORY PASADENA CALIF
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The anomalous weakness of the F 1 to 1 hyperfine component in the J 1 to 0 emission of interstellar HCN can be caused by radiative trapping in the J 2 to 1 lines. The anomaly is readily produced if the J 1 levels are populated largely by collisional excitation from J 0 to J 2 followed by radiative decay to J 1 with the J 2 to 1 lines optically thick. Regions where the anomaly is found probably have H2 density less than 10000cc and optical depths in the J 1 to 0 lines greater than 100.
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