The photo-ionization of He I (2 3 S) in nebulae.

Results are given for the destruction of metastable helium (He I, 2s3S) in gaseous nebulae by line and continuum radiation. Important contributors to this process include the stellar continuum shortward of 2600 A and trapped L-alpha line photons. Analytical formulas are given to calculate the importance of these effects from the stellar magnitude and nebular angular size, and from the dust-to-gas ratio. Photoionization models of a representative set of planetary nebulae and H II regions are used to evaluate the effect accurately, with careful attention paid to the treatment of ionization by L-alpha and C IV 1550 A photons, and to resonances in the photoionization cross-section of 2s3S. It is shown that photoionization produces significant destruction of metastable helium in compact, optically thick planetaries. The effect can reduce the 23S population by up to 25 percent there. In large, optically thin planetaries, and in giant extragalactic H II regions, the effect is very small and can be neglected.