2021MNRAS.502.4597H

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 Lyβ line (1022-1029 Å) to red visual light in the wings of the Hα line (6400 -6700 Å). Analysis of spatially resolved spectra of the Orion Bar and other regions in the Orion Nebula shows that this process occurs in the neutral photodissociation 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) ~10⁵ cm^–3 for the Orion Bar. Far-ultraviolet resonance lines of neutral oxygen imprint their absorption on to the stellar continuum as it passes through the ionization front, producing characteristic absorption lines at 6633 Å and 6664 Å with widths of order 2 Å. This is a unique signature of Raman scattering, which allows it to be easily distinguished from other processes that might produce broad Hα wings, such as electron scattering or high-velocity outflows.