Resonant Scattering of the He I 1.0833$μ$m Triplet in H II Regions: Emission Spectra
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arXiv
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| Format: | Preprint |
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2026
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| _version_ | 1866909998058045440 |
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| author | Draine, B. T. |
| author_facet | Draine, B. T. |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2601_11710 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Resonant Scattering of the He I 1.0833$μ$m Triplet in H II Regions: Emission Spectra Draine, B. T. Astrophysics of Galaxies 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. |
| title | Resonant Scattering of the He I 1.0833$μ$m Triplet in H II Regions: Emission Spectra |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2601.11710 |