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.
