AAS 207th Meeting, 8-12 January 2006
Session 81 ISM: From Dark to Blinded
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

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[81.17] Physical Conditions in Orion's Veil: A Multi Component Study of the Line of Sight Towards the Trapezium

N. P. Abel, G. J. Ferland, G. Shaw, T. H. Troland (University of Kentucky), C. R. O'Dell (Vanderbilt University)

Orion’s Veil is an absorbing screen that lies along the line of sight to the Orion H II region. It consists of two or more layers of gas that must lie within a few parsecs of the Trapezium cluster. Our previous work considered the Veil as a whole and found that the magnetic field dominates the energetics of the gas in at least one component. Here we use high-resolution STIS UV spectra that resolve the two velocity components in absorption and determine the conditions in each. We derive a volume hydrogen density, 21 cm spin temperature, turbulent velocity, and kinetic temperature, for each. We combine these estimates with magnetic field measurements to find that magnetic energy significantly dominates turbulent and thermal energies in one component, while the other component is close to equipartition between turbulent and magnetic energies. We observe molecular hydrogen absorption for highly excited v, J levels that are photoexcited by the stellar continuum, and detect blueshifted S III and P III lines. These ions must arise from ionized gas between the mostly neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We find that this layer of ionized gas is also responsible for He I 3889Å absorption towards the Veil, which resolves a 40-year-old debate on the origin of He I absorption towards the Trapezium. Finally, we determine that the ionized and mostly atomic layers of the Veil will collide in about 100,000 years

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