Resonant Scattering of the He I 1.0833$μ$m Triplet in H II Regions: Emission Spectra

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, multi-peaked 1.0833$μ$m emission profiles, with the centroid blue-shifted by up to $\sim$14 km/s relative to other lines. The feature FWHM can exceed 100 km/s for some regions. Resonant trapping enhances dust absorption and reduces the He I 1.0833$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 M-17B and NGC3603 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.