Raman mapping of photodissociation regions
  Broad Raman-scattered wings of hydrogen lines can be used to map neutral gas
illuminated by high-mass stars in star forming regions. Raman scattering
transforms far-ultraviolet starlight from the wings of the Lyman beta line
(1022 Angstrom to 1029 Angstrom) to red visual light in the wings of the H
alpha line (6400 Angstrom to 6700 Angstrom). Analysis of spatially resolved
spectra of the Orion Bar and other regions in the Orion Nebula shows that this
process occurs in the neutral photo-dissociation region between the ionization
front and dissociation front. The inner Raman wings are optically thick and
allow the neutral hydrogen density to be determined, implying n(H0) ~= 100,000
per cubic cm for the Orion Bar. Far-ultraviolet resonance lines of neutral
oxygen imprint their absorption onto the stellar continuum as it passes through
the ionization front, producing characteristic absorption lines at 6633
Angstrom and 6664 Angstrom with widths of order 2 Angstrom. This is a unique
signature of Raman scattering, which allows it to be easily distinguished from
other processes that might produce broad H alpha wings, such as electron
scattering or high-velocity outflows.
