THE MOLECULAR GAS ASSOCIATED WITH THE ORION BRIGHT BAR

Detailed studies of the shocked region associated with the Orion brigh t bar are presented. The millimeter wave spectral lines of (CO)-C-12 ( J = 1-0), (CO)-C-13 (J = 1-0), CS (J = 2-1), HCO+ (J = 1-0), and H51al pha have been observed across the bright bar. The intensity of all the molecular species shows a rapid increase close to the ionization fron t and a significant falloff at a distance of approximately 50'' farthe r out from it. This suggests the existence of a layer of dense molecul ar gas just outside the ionization front. This layer has a velocity re dshifted by 1-2 km s-1 relative to the ambient molecular cloud, which can be due to acceleration by thermal and kinetic pressure from the H II region or due to passage of a shock. The high-density molecular lay er associated with the bar is probably a shock-compressed layer driven by the ionization front of M42. A multitransitional analysis of the C S emission shows that the H-2 volume density of this molecular gas is larger than that of the ambient gas by a factor of 3. The apparent den sity enhancement of a factor of 3 in the shocked gas is too small for a radiative shock in a homogeneous medium; density inhomogeneities or clumpiness in the pre- and postshocked layer may account for this appa rently small compression ratio. This layer is exposed to intense UV ra diation from the Trapezium stars and a photodissociated region is form ed between the neutral layer and the ionization front. The similarity in distribution of the thermally excited H-2 emissions arising from sh ock fronts and/or dense photodissociation regions and the millimeter-w ave molecular line emissions originating from cooled shock-compressed regions also supports the idea of inhomogeneity or clumpiness in the s hocked cloud. The gas temperature of this shocked layer is about 100 K and is very high compared with other molecular clouds without an embe dded heat source. Both shock heating and radiative heating may contrib ute to maintain this high temperature.