The Near-Infrared Structure and Spectra of the Bipolar Nebulae M 2--9 and Afgl 2688: The Role of UV-Pumping and Shocks in Molecular Hydrogen Excitation

High-resolution near-infrared images and moderate resolution spectra were obtained of the bipolar nebulae M~2–9 and AFGL 2688. The ability to spatially and spectrally resolve the various components of the nebulae has proved to be important in determining their physical structure and characteristics. In M~2–9, the lobes are found to have a double-shell structure. Analysis of \h2
line ratios indicates that the \h2\ emission is radiatively excited. A well-resolved photodissociation region is observed in the lobes. The spectrum of the central source is dominated by H recombination lines and a strong continuum rising towards longer wavelengths consistent with a $T=795$ K blackbody. In AFGL 2688, the emission from the bright lobes is mainly continuum reflected from the central star. Several molecular features from C${}_2$ and CN are present. In the extreme end of the N lobe and in the E equatorial region, the emission is dominated by lines of \h2 in the 2–2.5 \microns region. The observed \h2 line ratios indicate that the emission is collisionally excited, with an excitation temperature $T_{ex}\approx 1600\pm 100$ K.