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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
We present new determinations of O^+2/H^+ values for several planetary nebulae based on ratios of O II and H I recombination lines. These line ratios are nearly independent of the electron temperature, and therefore in principle should yield more accurate abundances than the collisionally excited [O III] lines, which are very sensitive to temperature and to the presence of temperature structure and inhomogeneities. The observations were made with a CCD spectrograph on the McDonald Observatory 2.\@7m Harlan Smith telescope, with a spectral resolving power of about R = 1700 and spectral coverage from 4000-4700 Åa region which contains many of the strongest O II recombination lines. Measurements of at least one usable O II line were obtained for 14 nebulae, with 5 or more O II lines measured for six objects. We find average values of log(O^+2/H^+) + 12 = 9.1 -- 9.6, which is a factor of 1.5 -- 5 times greater than the solar O/H abundance of 8.9, and a factor of 3 -- 10 times greater than the typical nebular value of 8.6 which is derived from the [O III] lines. We discuss possible reasons for this discrepancy, including uncertainties in the line fluxes, uncertainties in the O II recombination coefficients, optical depth effects, and inhomogeneities in the physical conditions. We estimate the amplitude of the temperature fluctuations that would be required to bring the recombination-line O/H values into agreement with the forbidden-line O/H values.
Program listing for Wednesday