Physical Conditions in Orion’s Veil. II. A Multicomponent Study of the Line of Sight toward the Trapezium

Orion's Veil is an absorbing screen that lies along the line of sight to theOrion H II region. It consists of two or more layers of gas that must liewithin a few parsecs of the Trapezium cluster. Our previous work considered theVeil as a whole and found that the magnetic field dominates the energetics ofthe gas in at least one component. Here we use high-resolution STIS UV spectrathat resolve the two velocity components in absorption and determine theconditions in each. We derive a volume hydrogen density, 21 cm spintemperature, turbulent velocity, and kinetic temperature, for each. We combinethese estimates with magnetic field measurements to find that magnetic energysignificantly dominates turbulent and thermal energies in one component, whilethe other component is close to equipartition between turbulent and magneticenergies. We observe molecular hydrogen absorption for highly excited v, Jlevels that are photoexcited by the stellar continuum, and detect blueshifted SIII and P III. These ions must arise from ionized gas between the mostlyneutral portions of the Veil and the Trapezium and shields the Veil fromionizing radiation. We find that this layer of ionized gas is also responsiblefor He I absorption in the Veil, which resolves a 40-year-old debate on theorigin of He I absorption towards the Trapezium. Finally, we determine that theionized and mostly atomic layers of the Veil will collide in less than 85,000years.Comment: 43 pages, 15 figures, to be published in Ap