2001ApJ...559..985R

We present Heinrich Hertz Telescope observations in the N=3⟶2 rotational transition of the CN radical toward selected positions of the Trapezium region and of the molecular ridge in the Orion molecular cloud. Two of the positions in the ridge were also observed in the N=2⟶1 line of CN and ¹³CN. The N=3⟶2 CN lines have been combined with observations of the N=2⟶1 and N=1⟶0 transitions of CN and observations of the N=2⟶1 of ¹³CN in order to estimate the physical conditions and CN abundances in the molecular gas. We analyze in detail the excitation of the CN lines and find that the hyperfine ratios of the N=3⟶2 line are always close to the LTE optically thin values even in the case of optically thick emission. This is due to different excitation temperatures for the different hyperfine lines. From the line-intensity ratios between the different CN transitions we derive H₂ densities of ∼10⁵ cm^–3 for the molecular ridge and densities of ∼3x10⁶ cm^–3 for the Trapezium region. The CN column densities are 1 order of magnitude larger in the ridge than in the Trapezium region, but the CN to H₂ ratio is similar in both the Trapezium and the ridge. The combination of the low CN column densities, high H₂ densities, and relatively high CN abundances toward the Trapezium region requires that the CN emission arise from a thin layer with a depth along the line of sight of only ∼5x10¹⁵ cm. This high-density thin layer of molecular gas seems to be related to material that confines the rear side of the H II region Orion A. However, the molecular layer is not moving as expected from the expansion of the H II region but is ``static'' with respect to the gas in the molecular cloud. We discuss the implication of a high-density ``static'' layer in the evolution of an H II region.