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

Abstract

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 Å and trapped Lα line photons. Analytical formulae are given to calculate the importance of these effects from the stellar magnitude and nebular angular size, and from the dust-to-gas ratio. Photo-ionization models of a representative sci of planetary nebulae and H II regions are used to evaluate the effect accurately, with careful attention paid to the treatment of ionization by Lα and C IVλ1550 Å photons, and to resonances in the photo-ionization cross-section of 2s3S. It is shown that photo-ionization produces significant destruction of metastable helium in compact, optically-thick planetaries. The effect can reduce the 23S population by up to 25 per cent there. In large, optically-thin planetaries, and in giant extragalactic H II regions, the effect is very small and can be neglected. The line transfer problem for λ10830 Å radiation is solved and the destruction of line photons by dust absorption is evaluated. The results depend on the size distribution of the dust grains – which is uncertain – but destruction can reach 15 per cent for distributions deficient in small grains. It is noted that the optical depths in λ10830 deduced from self-absorption effects in the triplet lines are not consistent with the optical depths implied by the λ10830 line fluxes. The need for a further destruction mechanism for He I (23S) in nebulae is assessed.