The Orion bright bar is a prominent ionization front located approximately 2’ southeast of the Trapezium stars. Because this ionization front is seen almost edge-on, it provides an opportunity to study the interaction between the HII region and the adjacent molecular cloud. The molecular bar has been thought to be a narrow layer of ~ 50” (0.1 pc) in width parallel to the ionization front with enhanced temperature, density and column density. The molecular gas outside the ionization front was redshifted with respect to the ambient molecular cloud by 1-2 kms−1 (Omodaka et al. 1984, 1986, 1992), suggesting that the expanding HII region generated by the Trapezium stars had driven a shock wave into the molecular cloud at the southeast of the bar. This layer is exposed to intense UV radiation from the Trapezium stars, resulting in the formation of photodissociated regions.

We have made aperture synthesis observations of CS(J=1-0) line and 49 GHz continuum in the Orion bright bar with the Nobeyama Millimeter Array. Figure 1, a map of integrated intensities of CS, clearly revealed fine structures of the molecular bar and more than six prominent features are confirmed. It is noted that these features are lined up at 30” from the ionization front inside the molecular cloud.