Radiative trapping and hyperfine structure: HCN

The anomalous weakness of the F = 1 → 1 hyperfine component in the J = 1 → 0 emission of interstellar HCN can be caused by radiative trapping in the J = 2 → 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 → 1 lines optically thick. Regions where the anomaly is found probably have H_2 densities less than 10s^5 cm^(-3) and optical depths in the J =1 → 0 lines greater than 50.