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 Lyman beta line (1022 Angstrom 1029 Angstrom) red visual light H alpha (6400 6700 Angstrom). Analysis spatially resolved spectra Orion Bar and other regions Nebula shows that this process occurs photo-dissociation region between ionization front dissociation front. The inner are optically thick allow density determined, implying n(H0) ~= 100,000 per cubic cm for Bar. Far-ultraviolet resonance oxygen imprint their absorption onto stellar continuum as it passes through front, producing characteristic at 6633 6664 with widths order 2 Angstrom. This is a unique signature scattering, which allows easily distinguished processes might produce broad wings, such electron or high-velocity outflows.