A NEW LOOK AT CARBON ABUNDANCES IN PLANETARY-NEBULAE - III - DDDM1, IC-3568, IC-4593, NGC-6210, NGC-6720, NGC-6826, AND NGC-7009

This paper is the third in a series reporting on a study of carbon abu ndances in a carefully chosen sample of planetary nebulae representing a large range in progenitor mass and metallicity. We make use of the IUE Final Archive database containing consistently reduced spectra to measure line strengths of C III] lambda 1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009. We combine the IUE data with line s trengths from optical spectra obtained specifically to match the IUE s lit positions as closely as possible, in order to determine values for the abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C II I] lambda 1909/C II lambda 4267 is found to be effective for merging U V and optical spectra when He II lambda 1640/lambda 4686 is unavailabl e. Our abundance determination method includes a five-level program wh ose results are fine-tuned by corrections derived from detailed photoi onization models constrained by the same set of emission lines. All ob jects appear to have subsolar levels of O/H, and all but one show N/O levels above solar. In addition, the seven planetary nebulae span a br oad range in C/O values. We infer that many of our objects are matter- bounded, and thus the standard ionization correction factor for N/O ma y be inappropriate for these PNs. Finally, we estimate C/O using both collisionally excited and recombination lines associated with C+2 and find the well-established result that abundances from recombination li nes usually exceed those from collisionally excited lines by several t imes.