THE HE I-2.06 MICRON TO BR-GAMMA RATIO IN 12 PLANETARY-NEBULAE

We present the results of low-resolution (R = 30-100) spectroscopic observations of 12 planetary nebulae between 2.0 and 2.4 mu m. The data were acquired with a slitless grism imaging system and hence measure emission from each nebula as a whole. The strongest nebular features in this bandpass are found to be He I 2.058 mu m and Br gamma 2.166 mu m, which appear in a ratio of He I/Br gamma ranging from less than 0.15 to 1.2. We have compared these measurements with photoionization calculations appropriate for planetary nebulae. The behavior of the He I is dictated in large part by the temperature T-eff of the central star and the nebular density n(e). Ratios of He I to Br gamma predicted for uniformly expanding nebulae tend to exceed observed values systematically at a given T-eff and n(e). This behavior can be plausibly accounted for by complex velocity structure within real nebulae.