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Session 97 - Planetary Nebulae.
Display session, Thursday, January 16
We report the results of a near-infrared spectroscopic survey of planetary nebulae at McDonald Observatory during 1986 - 1993, using a grating spectrometer based on a 1 \times 32-element InSb array. Most of the observations were taken in the K-band, with resolving power R = \lambda/\Delta \lambda = 600 in order to resolve v = 1-0 S 2.121\,\mum H_2 from the adjacent He\,\sc i 2.113\,\mum line. We also simultaneously measured the strengths of H\,\sc i Br \gamma and He\,\sc ii 2.189\,\mum. The typical limiting flux in our 3.8\arcsec square beam was 1 - 3 \times 10^-14 erg cm^-2 s^-1, corresponding to a limiting surface brightness of 3 \times 10^-5 to 1 \times 10^-4 erg cm^-2 s^-1 ster^-1. An early result of this study was the first conclusive proof that the UV-pumping or ``fluorescence'' mechanism rather than thermal (shock) excitation was responsible for the H_2 emission in a planetary nebula, Hubble 12 (Dinerstein et al. 1988, ApJ, 327, L27). Here we report results for 15 additional nebulae, 8 of which showed detectable H_2 emission. We mapped 6 objects by stepping the aperture along E-W and/or N-W axes through the central star, at intervals separated by 3\arcsec, in order to find a position which isolated the H_2 emission; observations at the central position or with large, centered beams are often contaminated by strong continuum and line emission from the central star and the ionized gas. For selected positions, we also obtained observations in the J and H bands and with a setting that included the 1-0 S(0) 2.223\,\mum and 2-1 S(1) 2.247\,\mum H_2 lines. The observed line intensity ratios range from those characteristic of ``pure fluorescence'' to values typical of thermally-excited (or thermalized fluorescent) emission, with the planetary nebula BD+30^\circ 3639 representing an intermediate case.
Program listing for Thursday