2006AJ....131.1515L

We present observations of 16 planetary nebulae (PNs) in the 2 µm (K band) spectral region, obtained with a long-slit near-infrared spectrometer at McDonald Observatory. In general, the strongest features in our spectra are recombination lines of H I, He I, and (in some cases) He II. Half the sample shows emission from vibrationally excited H₂. Some of the observed PNs (e.g., M 1-13) display H₂ line ratios characteristic of shocked, thermalized gas, while others (e.g., BD+30 3639) have ratios intermediate between pure radiative (UV) and shock excitation, consistent with either a combination of the mechanisms or UV illumination of dense material. Our spectra of J900 and M 1-13 confirm that published narrowband images trace the H₂ emission, and we find that the H₂ emission in SwSt 1 has a larger spatial extent than previously reported. In IC 5117, SwSt 1, and NGC 40 we detect the [Kr III] 2.199 µm line identified by Dinerstein in 2001, with strengths indicating that krypton is enriched relative to the solar abundance, most markedly so in NGC 40. We also detect several lines from the ³G term of [Fe III] in Vy 2-2, SwSt 1, and marginally in Cn 3-1. The [Kr III] and [Fe III] lines fall near in wavelength to H₂ transitions, which are often used as diagnostics for UV excitation because they arise from higher vibrationally excited levels (v=2, 3). For moderate spectral resolving power, R≤600, these lines may be blended with, or even mistaken for, the corresponding H₂ lines, leading to misinterpretation of the H₂ emission. The strength of both the Kr and Fe nebular emission lines can be enhanced by special circumstances, Kr because of nucleosynthetic self-enrichment in the progenitor star and Fe due to inefficient initial dust condensation or partial destruction of the dust after formation, causing a larger fraction of the elemental iron to reside in the gas phase.