COLLISIONAL EFFECTS IN HE-I - AN OBSERVATIONAL ANALYSIS

Accurate and reliable helium abundances can test modern theories of ga lactic and primordial nucleosynthesis. Unfortunately, there is now som e question whether current theory can account for collisional contribu tions to He I. We present new observations of two planetary nebulae (P Ns) in the range lambda 3850-lambda 9650, which we use to assess the i mportance of collisional effects in the He I spectrum. The first objec t, NGC 7027, is expected to show relatively strong collisional enhance ment, while the second, NGC 7026, should display only small effects. W e derive new collision-to-recombination correction factors, based on n ew collision strengths from a 29-state quantal calculation of He I ext ending 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 obse rvational evidence for the existence of an unknown agent depopulating the 2(3)S State in PNs.