ar X iv : a st ro - p h / 04 01 04 5 v 1 6 J an 2 00 4 12 C / 13 C ratio in planetary nebulae from the IUE archives

We investigated the abundance ratio of C/C in planetary nebulae by examining emission lines arising from C III 2s2p P2,1,0 → 2s 2 S0. Spectra were retrieved from the International Ultraviolet Explorer archives, and multiple spectra of the same object were coadded to achieve improved signal-to-noise. The C hyperfine structure line at 1909.6 Å was detected in NGC 2440. The C/C ratio was found to be ∼4.4±1.2. In all other objects, we provide an upper limit for the flux of the 1910 Å line. For 23 of these sources, a lower limit for the C/C ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high signal-to-noise C III spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 flux ratio outside the low-electron density theoretical limit for C. A mixture of C with C helps to close the gap somewhat. Nevertheless, some observed 1907/1909 flux ratios still appear too high to conform to the presently predicted limits. It is shown that this limit, as well as the 1910/1909 flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet S0– PJ according to the statistical weights. A detailed calculation for the fine structure collision strengths between these individual levels would be valuable. Subject headings: Atomic processes — planetary nebulae: abundances, general, individual (NGC 2440) — stars: abundances, AGB and post-AGB NASA/Ames Research Center, Moffett Field, CA 94035-1000. Orion Enterprises, M.S. 245-6, Moffett Field, CA 94035-1000 University of Kentucky, Department of Physics and Astronomy, Lexington, KY 40506.