FABRY-PEROT IMAGING OF H_2 1-0 S(1), 2-1 S(1), AND BRACKETT-GAMMA EMISSION IN THE ORION NEBULA

摘要

We obtained spectral maps of the central 6′ x 8′ (0.9 pc x 1.2 pc) region of the Orion Nebula. H_2 v = 1-0 S(1), 2-1 S(1) [S(1) means J = 3-1], and Brγ maps were obtained with the wide field Fabry-Perot imager at the Nasmyth focus of the Communications Research Laboratory 1.5 m telescope, FINAC. In the Bright Bar, a typical photodissociation region (PDR), the H_2 2-1 S(1)/1-0 S(1) line ratio (≡ R_(2-1/1-0)) is between 0.2 and 0.6 and has a tendency to be anticorrelated to the intensity of H_2 1-0 S(1) [≡ I_(1-0 S(1))]. From a comparison of the observed R_(2-1/1-0) versus I_(1-0 S(1)) relation with theoretical models, we conclude that the H_2 thermal component in this region is due not to shock heating caused by the expansion of the H II region, but to the collisional de-excitation of the lower vibrational levels populated by cascade after UV pumping. Our results strongly suggest that the Bright Bar region is a PDR consisting of a medium density (～10~4-10~5 cm~(-3)) cloud and a small number of denser (approx> 10~6 cm~(-3)) clumps whose size is smaller than our spatial resolution (8″ = 0.02 pc). We also find that the "south region" and "east region" of the Orion Nebula can be described as PDRs similar to the Bright Bar region.