${}^{12}$C/${}^{13}$C 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 \ion{C}{3} $2s2p ^3P_{2,1,0} \to 2s^2 ^1S_0$. 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 ${}^{13}$C hyperfine structure line at 1909.6 Åwas detected in NGC 2440. The ${}^{12}$C/${}^{13}$C ratio was found to be $\sim 4.4\pm$1.2. In all other objects, we provide an upper limit for the flux of the 1910 Åline. 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 \ion{C}{3} 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 ${}^{12}$C. A mixture of ${}^{13}$C with ${}^{12}$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 ${}^1{S}_0$--${}^3{P}_J$ according to the statistical weights. A detailed calculation for the fine structure collision strengths between these individual levels would be valuable.

ApJ accepted: 19 pages, 3 Figures, 2 Tables