Raman mapping of photodissociation regions

TL;DR

This paper demonstrates how Raman scattering of far-ultraviolet starlight can be used to map and analyze neutral gas in star-forming regions, providing insights into hydrogen density and unique spectral signatures.

Contribution

It introduces a novel application of Raman scattering to map neutral gas in PDRs and identifies distinctive spectral features for this process.

Findings

Raman wings occur in the neutral PDR between ionization and dissociation fronts.

Inner Raman wings are optically thick, enabling hydrogen density estimation.

Characteristic absorption lines of neutral oxygen serve as signatures of Raman scattering.

Abstract

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.