12C/13C ratio in planetary nebulae from the IUE archives

We investigated the abundance ratio of C-12/C-13 in planetary nebulae by examining emission lines arising from C III 2s2p P-3(2,1,0)o --> 2s(2) S-1(0). Spectra were retrieved from the International Ultraviolet Explorer archives, and multiple spectra of the same object were co-added to achieve improved signal-to-noise ratio. The C-13 hyperfine structure line at 1909.6 Angstrom was detected in NGC 2440. The C-12/C-13 ratio was found to be similar to 4.4 +/- 1.2. In all other objects, we provide an upper limit for the flux of the 1910 Angstrom line. For 23 of these sources, a lower limit for the C-12/C-13 ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high-signal-to-noise ratio C III spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 Angstrom flux ratio outside the low - electron density theoretical limit for C-12. A mixture of C-13 with C-12 helps to close the gap somewhat. Nevertheless, some observed 1907/1909 Angstrom flux ratios still appear too high to conform to the currently predicted limits. It is shown that this limit, as well as the 1910/1909 Angstrom flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet S-1(0) - P-3(J)o according to the statistical weights. A detailed calculation for the fine-structure collision strengths between these individual levels would be valuable.