The Infrared Spectra of Planetary Nebulae

We present results from three projects on the infrared spectra of planetary nebulae. In the first project we surveyed the lambda = 0.9 - 1.3\ microns spectra of a sequence of objects ranging from Mira variables through evolved planetary nebulae. We find that the appearance of the spectra is determined primarily by the temperature of the exciting star. Shocks are important to the excitation and shaping of the nebulae. Clumps of relatively high density gas appear to be common features. We also find that the strengths of low ionization and molecular features decrease with age. This change comes about because of a decrease in the amount of low ionization material close to the star. Our second project is a search for extended, fluorescent H_2 emission around planetary nebulae. We made large beam (45('') ) measurements of the H_2 v = 1 - 0 S(1), 2 -- 1 S(1), and 6 -- 4 Q(1) lines in the near-infrared spectra of J900, AFGL 618, and NGC 2346. We find evidence for UV excitation of the molecule in the ``collisional fluorescence'' regime, but there is no indication of extended, radiatively excited H_2 emission. Our third project is a search for lines from highly ionized species in the mid-infrared spectrum of NGC 7027. Our main goal is to determine accurate wavelengths for these lines, although we also plan to use the lines to study excitation and abundances in the nebula.