Physical conditions in the Orion HII region

The Orion Nebula is the defining Galactic H ii region and has been well studied over a broad range of wavelengths. Many of its properties are well established. The gas- phase metallicity is roughly two- thirds of solar, close to that of the star cluster. There is no evidence that grains are destroyed anywhere in the Orion environment; refractory elements such as Ca and Fe remain heavily depleted. If the grains are well mixed with the gas, then the Trapezium cluster's radiation field will drive the gas away, because of the grain opacity. This pushes the grain- bearing gas back against the photodissociation region (PDR). Radiation pressure is more important than winds in controlling the overall geometry. 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 (which feel the force of the radiation field) to the gas (which has a far lower opacity but most of the mass). The grains drift relative to the gas, with (model dependent) speeds of order a fraction of a km s(-1). The H ii region has a physical thickness of roughly 10(12) km. The grains can drift an appreciable fraction of the thickness of the ionized layer in a timescale of t < 10(13) s <approximate to> 10(5) yr. The age of the current geometry is unknown since the motion of theta (1) Ori C is unknown. The likely equilibrium is one where grains drift away from the illuminated face of the H ii region and accumulate near the PDR. The ionized gas, having lost its opacity, then expands into the direction of the central star cluster, loses its emission measure, and eventually becomes invisible. The loss of photoelectric grain heating in parts of the H ii region may cause a dramatic change in electron temperature and provide a natural explanation for the "t(2)" or temperature fluctuations phenomenon. The role of the magnetic field in controlling properties of the H ii region is unclear, although a component of turbulence equal to the Alfven speed has been seen. Clearly much work remains to be done.