Capture-Cascade Intensities of the Helium Singlets in Nebulae

Effective recombination coefficients and recombination intensities of the helium singlets have been calculated for a range of temperatures and densities appropriate to diffuse and planetary nebulae (T = 5000 , 100000, and 200000 K; N, in the range 0-10'). The equilibrium equations for the 210 lowest singlet levels (n, 1; n < 20) have been solved simultaneously and the effect of cascades from levels with n > 20 has been approximately allowed for, as has the effect of collisional (n, IH It, t 11) interactions. Comparison with observed intensities in nebulae indicates that agreement is satisfactory for those singlet lines most unaffected by self-absorption from the ground , n 1D-2 iP. Other series are observed to be stronger than recombination theory predicts, presumably due to the degradation of optically thick Lyman lines. The intensity ratios of triplets to singlets are!in satisfactory agreement with these calculations. Although It = 2 singlet populations are significantly altered by interactions with 2 3S, this has little effect on observed line intensities. Temperature effects in the spectra are large enough to be measured only for emissions originating in S states; density effects in the spectra are small.