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

We investigated the abundance ratio of ¹²C/¹³C in planetary nebulae by examining emission lines arising from C III 2s2p ³P_2,1,0⁰ → 2s^{2 1}S₀. 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 ¹³C hyperfine structure line at 1909.6 Å was detected in NGC 2440. The ¹²C/¹³C 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 ¹²C/ ¹³C ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high-signal-to-noise ratio C in spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 Å flux ratio outside the low-electron density theoretical limit for 12^C. A mixture of ¹³C with ¹²C helps to close the gap somewhat. Nevertheless, some observed 1907/1909 Å 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 Å flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet ¹S₀ - ³P_J⁰ according to the statistical weights. A detailed calculation for the fine-structure collision strengths between these individual levels would be valuable.