He I 2.06 micron emission from nebulae

The spectrum emitted by any astronomical plasma is sensitive to a variety of details, some of which may not be obviously important. This paper describes the sensitivity of the He I 2.06 μm line to the gas opacity at ionizing energies. The intensity of the line relative to a hydrogen line depends on the He ⁺/H⁺ ratio, but also on the ratio of continuous to He I Lyα line opacity, since this determines whether the Lyα line can scatter often enough to be converted to the 2.06 μm line. The intensity of the infrared line relative to Hβ can change by factors of several depending on details of the radiative transfer of He I Lyα, the gas microturbulence, the dust-to-gas ratio, and the level of ionization.