Resonant Scattering of the He i 1.0833 μ m Triplet in H ii Regions: Emission Spectra

Abstract Resonant scattering of He i 1.0833 μ m triplet photons by metastable He 2 3 S 1 is studied for optical depths characteristic of H ii regions. Regions with large He 2 3 S 1 column densities are predicted to have unusually broad multipeaked 1.0833 μ m emission profiles, with the centroid blueshifted by up to ∼14 km s −1 relative to other lines. The feature FWHM can exceed 100 km s −1 for some regions. Resonant trapping enhances dust absorption and reduces the He i 1.0833 μ m emission. Care must be taken when using the He i 1.0833 μ m/H i 1.0941 μ m (Pa γ ) ratio to estimate the He + /H + ratio. Predicted spectra are computed for examples, including M17-B and NGC 3603 in the Galaxy, and a star-forming region in M51. Observations of the 1.0833 μ m triplet with spectrometers such as NIRSPEC, CARMENES, or X-Shooter can confirm the predicted effects of resonant scattering in H ii regions, and constrain the nebular conditions.