2026ApJ...999....3D

Resonant scattering of He I 1.0833 μm triplet photons by metastable He 2 ³S₁ is studied for optical depths characteristic of H II regions. Regions with large He 2 ³S₁ column densities are predicted to have unusually broad multipeaked 1.0833 μm 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 μm emission. Care must be taken when using the He I 1.0833 μm/H I 1.0941 μm (Paγ) 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 μm 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.