A three-dimensional PDR model shows that overlapping H2 dissociation and C+ recombination fronts, plus the arc-like H2 emission seen in the Orion Bar, arise from shadowing by dense clumps in a fractal gas distribution.
H_2 Pure Rotational Lines in the Orion Bar
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Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H_2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2" resolution. We also observed H_2 v = 0-0 S(4) at selected points toward the front of the PDR. Our maps cover a 12" by 40" region of the bar where H_2 ro-vibrational lines are bright. The distributions of H_2 0-0 S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail. The high spatial resolution (0.002 pc) of our observations allows us to probe the distribution of warm gas in the Orion Bar to a distance approaching the scale length for FUV photon absorption. We use these new observational results to set parameters for the PDR models described in a companion paper (Draine et al. 2005, in prep). The best-fit model can account for the separation of the H_2 emission from the ionization front and the intensities of the ground state rotational lines as well as the 1-0 S(1) and 2-1 S(1) lines. This model requires significant adjustments to the commonly used values for the dust UV attenuation cross section and the photoelectric heating rate.
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Resolving dense photo-dissociation regions: the structure of photochemical fronts in three-dimensional gas distributions
A three-dimensional PDR model shows that overlapping H2 dissociation and C+ recombination fronts, plus the arc-like H2 emission seen in the Orion Bar, arise from shadowing by dense clumps in a fractal gas distribution.