Orion's Veil: Magnetic field strengths and other properties of a PDR in front of the Trapezium Cluster

We present an analysis of physical conditions in the Orion Veil, a largely atomic PDR that lies just in front (about 2 pc) of the Trapezium stars. We have obtained 21 cm HI and 18 cm OH VLA Zeeman effect data. These data yield images of the line-of-sight magnetic field strength Blos in atomic and molecular regions of the Veil. We find Blos is typically -50 to -75 microgauss in the atomic gas across much of the Veil (25" resolution); Blos is -350 microgauss at one position in the molecular gas (40" resolution). The Veil has two principal HI velocity components. Magnetic and kinematical data suggest a close connection between these components. They may represent gas on either side of a shock wave preceding a weak-D ionization front. Magnetic fields in the Veil HI components are 3-5 times stronger than they are elsewhere in the ISM where N(H) and n(H) are comparable. The HI components are magnetically subcritical (magnetically dominated), like the CNM, although they are about 1 dex denser. Strong fields in the Veil HI components may have resulted from low turbulence conditions in the diffuse gas that gave rise to OMC-1. Strong fields may also be related to magnetostatic equilibrium that has developed in the Veil since star formation. We consider the location of the Orion-S molecular core, proposing a location behind the main Orion H+ region.