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

Abstract

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.