Fabry-Perot Imaging of H 2 1--0 S(1), 2--1 S(1), and Brackett-Gamma Emission in the Orion Nebula

Details Fabry-Perot Imaging of H 2 1--0 S(1), 2--1 S(1), and Brackett-Gamma Emission in the Orion Nebula Fabry-Perot Imaging of H 2 1--0 S(1), 2--1 S(1), and Brackett-Gamma Emission in the Orion Nebula

Jun 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...464..818u&link_type=abstract

Astrophysical Journal v.464, p.818

Astronomy and Astrophysics

Astronomy

22

Ism: H Ii Regions, Infrared: Ism: Lines And Bands, Ism: Individual Name: Orion Nebula, Ism: Molecules

Scientific paper

We obtained spectral maps of the central 6' × 8' (0.9 pc × 1.2 pc) region of the Orion Nebula. H2 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 H2 2-1 S(1)/1-0 S(1) line ratio (≡ R2-1/1-0) is between 0.2 and 0.6 and has a tendency to be anticorrelated to the intensity of H2 1-0 S(1) [≡ I1-0 S(1)]. From a comparison of the observed R2-1/1-0 versus I1-0 S(1) relation with theoretical models, we conclude that the H2 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 (˜104-105 cm-3) cloud and a small number of denser (≥106 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.
In the Orion KL region, a typical shocked gas of a bipolar outflow source, we find that R2-1/1-0 is independent of I1-0 S(1). R2-1/1-0 is ˜0.08 near the central area of KL, the curved bridgelike structure, and the finger-like filaments of the Orion KL region ("finger region"). On the other hand, R2-1/1-0 is ˜0.05 at the edge of the central area. The difference in the ratio suggests that high-velocity shocks exist at the center of KL, while low-velocity shocks surround the central area. The Orion S region contains some thermal H2 clumps which may be shock excited. The distribution of these shocked-H2 clumps, CO J = 2-1 clumps, and far-infrared sources suggests the existence of at least two sets of bipolar outflows associated with young stellar objects in this area.
In a global view of the Orion Nebula, the compact distribution of Brγ relative to H2 1-0 S(1) indicates the existence of a PDR surrounding the H II region. Moreover the intensity ratio H2 1-0 S(1) from PDRs (R2-1/1-0 ≥ 0.15; i.e., the PDR including dense molecular clumps) to Brγ (≡R1-0(PDR)/Brγ) is 0.17. By comparing this value for Orion to those for starburst galaxies (cf. R1-0/Brγ = 0.4-0.9), we conclude that the typical starburst region has larger H2-emitting areas surrounding H II regions and/or a later average spectral type for exciting stars than the Orion Nebula, if the H2 emission from starburst galaxies is predominantly fluorescent.

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