OBSERVATIONS OF THE BOWEN FLUORESCENCE MECHANISM AND CHARGE-TRANSFER IN PLANETARY NEBULAE-I

Deep, medium-resolution, long-slit spectrophotometric data have been taken for a number of high-excitation planetary nebulae, covering a wavelength range from 3100 to 7200 angstrom with some selected regions observed at higher resolution. For about half the objects, the whole optical region has been observed, from 3100 to 11 600 angstrom. Accurate flux calibration is achieved over this whole wavelength range. These data allow a detailed quantitative study of the Bowen fluorescence mechanism and the charge transfer reaction O3+(2P3/2,1/2)+H0 --> O2+(2s'(a)+1L'(a,j(a))+H+ in planetary nebulae, the primary goal of this programme. In this paper measurements of O2+ Bowen fluorescence and charge transfer lines are presented. We show that LS coupling fails for the O2+ permitted transitions studied in this programme and intermediate coupling may be a better assumption. Efficiencies of the Bowen fluorescence mechanism are derived for 15 objects, and a wide range of efficiencies is apparent. There is a remarkable linear positive correlation between the Bowen efficiency, the fractional abundance of oxygen in the ionization stage of O2+ and the fractional abundance of helium in the form of He+. Evidence that the Bowen efficiency is anti-correlated with the electron temperature, as first noted by Likkel & Aller, is established. The Bowen efficiency drops substantially when the nebular expansion velocity 2 V(exp)(O2+) > 55 km s-1. For lower expansion velocities there is no detectable correlation between the expansion velocity and the Bowen efficiency. No observable differences in Bowen efficiency among objects of different morphological type or between objects excited by stars of different spectral types as suggested by Likkel & Aller were seen. The O III lambdalambda 3261, 3265 and 3627 lines are found to originate from radiative and dielectronic recombination. Reliable O3+ abundances are derived for some objects using these lines.