Collisional effects in He I: An observational analysis

Accurate and reliable helium abundances can test modern theories of galactic and primordial nucleo-synthesis. Unfortunately, there is now some question whether current theory can account for collisional contributions to He I. We present new observations of two planetary nebulae (PNs) in the range λ3850-λ9650, which we use to assess the importance of collisional effects in the He I spectrum. The first object, NGC 7027, is expected to show relatively strong collisional enhancement, while the second, NGC 7026, should display only small effects. We derive new collision-to-recombination correction factors, based on new collision strengths from a 29-state quantal calculation of He I extending to n = 5. Our results show that the correction factors based on "standard" theory are correct, but that the errors acquired in such an observational analysis are appreciable. We find no convincing observational evidence for the existence of an unknown agent depopulating the 2 ³S state in PNs.