Hora, J.H., and Latter, W.B. 1994, ApJ, 437, 281.

The Near-Infrared Structure and Spectra of the Bipolar Nebulae M2-9 and AFGL 2688: The Role of Ultraviolet Pumping and Shocks in Molecular Hydrogen Excitation

William B. Latter
National Radio Astronomy Observatory - Tucson

1994, ApJ, 437, 281
 

High-resolution near-infrared images and moderate resolution spectra were obtained of the bipolar nebulae M2-9 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 M2-9, the lobes are found to have a double shell structure. The inner shell is dominated by emission from hydrogen recombination lines, and the outer shell is primarily emission from H₂ lines in the 2 - 2.5 micron region. 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 toward longer wavelengths consistent with a T = 795 K blackbody. Also present are lines of He I and Fe II. In contrast, the N knot and E lobe of M2-9 show little continuum emission. The N knot spectrum consists of lines of (Fe II) and hydrogen recombination lines. In AGFL 2688, the emission from the bright lobes is mainly continuum reflected from the central star. Several molecular features from C₂ 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 2.0 - 2.5 micron region. The observed H₂ line ratios indicate that the emission is collisionally excited, with an excitation temperature T(ex) approximately = 1600 +/- 100 K.

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