12C/13C Ratio in Planetary Nebulae from the IUE Archives

Abstract

We investigated the abundance ratio of 12 C/ 13 C in planetary nebulae by examining emission lines arising from C III 2 s 2 p 3 P → 2 s 2 1 S 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 13 C hyperfine structure line at 1909.6 Å was detected in NGC 2440. The 12 C/ 13 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 12 C/ 13 C 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 Å flux ratio outside the low-electron density theoretical limit for 12 C. A mixture of 13 C with 12 C 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 1 S 0 - 3 P according to the statistical weights. A detailed calculation for the fine-structure collision strengths between these individual levels would be valuable.