Physical Conditions in the Orion HII Region
  The Orion Nebula is the defining galactic HII Region, and has been well studied over a broad range of wavelengths. The gas-phase metallicity is 2/3 of solar, close to the star cluster. Grains survive throughout. The Trapezium cluster's radiation field will drive the dusty gas away, pushing it back against the photodissociation region (PDR). The nebula is in a state of energy equipartition between energy input from starlight and energy stored in gas pressure, kinetic outflow, and the magnetic field. There is much that is not clear, however. How can gas flow away from the PDR into the face of a radiation field that produces such effective radiative acceleration? Grains are photoionized by the stellar radiation field, and this charge establishes a Coulomb drag that couples the grains to the gas. The grains drift relative to the gas, with speeds of order a fraction of a km s^-1. The grains can drift an appreciable fraction of the thickness of the ionized larger in a timescale of roughly 10^5 yr. The age of the geometry is unknown since the motion of t1 C Ori is unknown. The likely equilibrium is one where grains drift away from the illuminated face of the HII region and accumulate near the PDR. The ionized gas then expands into the direction of the central star cluster, looses its emission measure, and eventually becomes invisible. The loss of photoelectric grain heating may cause a dramatic change in the temperature, and provide a natural explanation for the temperature fluctuations phenomenon. The role of the magnetic field in controlling properties of the HII region is unclear, although a component of turbulence equal to the Alfven speed has been seen. Much work remains to be done. 
Gary J. Ferland (CITA)