The motion and distribution of the vibrationally excited H2 in the Orion molecular cloud

Observations of the v=1-0 S(1) and S(0), and v=2-1 S(1) emission lines of H2 in the Orion molecular cloud are presented, showing that the emission region has an approximately circular symmetry, which may be divided into a central and a peripheral region. The emission lines in the central region have a large range of velocities, and predominantly occur at the negative velocities of 0 to -100 km/sec, while those at the periphery are symmetric. The brightness of the peak emission is generally higher at positions in the periphery than in the central region. These results lead to a model of H2 line emission in which there is gas undergoing radial expansion at velocities of up to about 100 km/sec within a region of some 2.5 x 10 to the 17th cm in diameter around the cluster of IR sources. A substantial part of the H2 line emission comes from the outside boundary of this expansion region, where the flow collides with the gas in the molecular cloud.