THE RICH O-II-RECOMBINATION SPECTRUM OF THE PLANETARY-NEBULA NGC-7009- NEW OBSERVATIONS AND ATOMIC DATA

We present new spectrophotometric observations of the rich O II optica l recombination line spectrum of the planetary nebula NGC 7009, obtain ed at a spectral resolution of about 1 Angstrom (FWHM). New intermedia te coupling quantal calculations of O II radiative recombination coeff icients for the 3d-3p and 4f-3d transitions are presented. The effect of departure from pure LS-coupling is shown to be important. Excellent agreement is found between the observed relative intensities of the O II lines and those calculated from recombination theory allowing for intermediate coupling effects. C, N and O abundances based on our reco mbination line measurements are derived. In all cases, they are about a factor of 5 higher than the corresponding values deduced from collis ionally excited lines, indicating that the discrepancy between the abu ndances derived from these two different types of emission Lines, prev iously known to exist for C2+, is a common phenomenon, and is probably caused by the same physical process. The nature of this process is st ill not known. If the discrepancy is due to temperature fluctuations, the implied rms temperature fluctuation parameter t(2) is about a fact or of 2 larger than that derived by comparing the temperatures deduced from the [O III] forbidden line ratio and from the ratio of the nebul ar continuum Balmer discontinuity to H beta. However, if we adopted th e electron temperature derived from nebular continuum Balmer discontin uity instead of that from the [O III] forbidden line ratio, the C and N abundances deduced from ultraviolet collisionally excited lines woul d come into agreement with those deduced from the optical recombinatio n lines, although the abundance of oxygen deduced from the optical for bidden lines would still be a factor of 2 lower than the corresponding value obtained from the optical recombination lines. The O/H abundanc e ratio derived from our recombination line analysis of NGC 7009 is mo re than a factor of 2 higher than the solar oxygen abundance.