The He I 2.06 micron to Br ratio in 12 planetary nebulae
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We present the results of low-resolution (R = 30-100) spectroscopic observations of 12 planetary nebulae between 2.0 and 2.4 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 m and Br 2.166 m, which appear in a ratio of He I/Br 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 Teff of the central star and the nebular density ne. Ratios of He I to Br predicted for uniformly expanding nebulae tend to exceed observed values systematically at a given Teff and ne. This behavior can be plausibly accounted for by complex velocity structure within real nebulae.