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2001PASP..113...41F - Publ. Astron. Soc. Pac., 113, 41-48 (2001/January-0)
Physical conditions in the Orion H II region.
FERLAND G.J.
Abstract (from CDS):
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 1012 km. The grains can drift an appreciable fraction of the thickness of the ionized layer in a timescale of t<1013 s~105 yr. The age of the current geometry is unknown since the motion of {thgr}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 ``t2'' 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 Alfvén speed has been seen. Clearly much work remains to be done.
Abstract Copyright: ∼
Journal keyword(s): ISM: Individual: Name: Orion
Simbad objects: 3
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