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N. P. Abel (University of Kentucky Department of Physics and Astronomy), C. L. Brogan (National Radio Astronomy Observatory), G. J. Ferland, T. H. Troland (University of Kentucky Department of Physics and Astronomy)
Orion's veil, a region of largely neutral gas that lies between us and the main ionizing stars of the Orion nebula, is one of the few regions where the magnetic field and extinction can be accurately mapped. The field is strong, and the column density of molecular hydrogen is at least 4 orders of magnitude lower than expected from the measured extinction, temperature, and magnetic field. By contrast the OH column density is consistent with a region that is predominately molecular. The veils distance away from the Trapezium, its density, and physical thickness are poorly known. Determining the density is a high priority since we could then compare the magnetic, thermal, and turbulent energy density, and determine whether magnetic or gas pressure dominates the region.
Here we compute photoionization models of the veil for a variety of gas densities and distances to the Trapezium. Our calculations almost always produce too much molecular hydrogen, although the predicted OH agrees with observations. The anomalously large hydroxyl to molecular hydrogen ratio suggests that the chemistry is in a non-equilibrium state, possibly caused by radiation from a recent supernova. Our best models have a density between 1 and 10 thousand atoms per cubic centimeter, suggesting that the veil is magnetically dominated. The inferred distance from the Trapezium is 0.5 - 2.0 parsecs, about 2-8 times farther away from the Trapezium than the main ionization front of the Orion nebula. Lastly, we find that about one-eighth of the total hydrogen column density along the line of sight to the Orion Nebula is ionized and so undetectable by absorption measurements. The ratio of visual extinction to hydrogen column density, Av/N(H), is 60% of the average value in the ISM. This lower Av/N(H) is consistent with the idea that the grain size distribution is dominated by larger grains and requires less material to account for the extinction.
We would like to thank the NSF and NASA for support in our research.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.