Physics
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........23k&link_type=abstract
Thesis (PhD). THE UNIVERSITY OF TEXAS AT AUSTIN, Source DAI-B 60/12, p. 6152, Jun 2000, 164 pages.
Physics
1
Scientific paper
Using our custom designed telescope, we mapped the emission of molecular hydrogen on unprecedented angular scales in the Orion A and B molecular clouds as well as the Galactic center. Our goal was to the study the structure and physics of photodissociation regions. By comparing the relative intensities of the H2 (1,0)S(1) lines with the H2 (6,4)Q(1) lines we observed in Orion A and B, we have determined that ultraviolet-pumped fluorescence is the dominant mechanism for this global emission. We verified this result further by comparing these H2 line intensifies, along with the FIR dust emission and the [C II] 158 μm line emission, with predictions from models. Modeling has also indicated that the surfaces of these molecular clouds are extremely dense with ultraviolet fields ranging between 10 and 1000 times the intensity of the mean interstellar radiation field. 13CO data from these regions indicate that the column densities of these clouds are relatively low implying that the molecular clouds are probably filamentary in structure. Comparing the line intensifies for the H2 (1,0)S(1) and H 2 (2,1)S(1) transitions we observed in the Galactic center reveals that ultraviolet excitation is responsible for the observed degree-scale emission and not shocks. Within the central 600 pc the emission is concentrated in a plane that is tipped by about 15° relative to the Galactic plane. Close to the bII = 0° plane the emission is concentrated between -1° <= lII <= +2°. Within the central 400 pc the scale height of H2 emission is about 0.56° (+0.15°/-0.08°). The total luminosity of the H2 (1,0)S(1) line over the surveyed region is about 6.4 × 104 Lsolar . Models show that the overall density of the emitting gas in the Galactic center is high and, except for our central beam, not high enough for the H 2 line ratios to show evidence of thermalization (n ~ 5 × 104 cm-3). Ultraviolet field strengths were found to be highest in the center (IUV ~ 1600) and gradually decreasing for increasing angular distances away from the center. When nearby H II regions were compared with the Galactic center on similar scales they were found to have densities that are about an order of magnitude lower and ultraviolet fields strengths that were comparable.
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