Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418

We present high signal-to-noise echelle spectra of the compact high surfacebrightness, low ionization planetary nebula IC 418. These reveal 807 emissionlines down to intensities less than 10$^{-5}$ that of H$\beta$ for which wedetermine widths and relative intensities. We show that line profiles are avaluable parameter for making line identifications and in constraining theexcitation mechanism of the lines. We present evidence that indicates that manysupposed high-level recombination lines may in fact be excited by a processother than recombination. We contend from the detection of dielectronicrecombination lines that their relatively low intensities argue against theirmaking a significant contribution to level populations of the heavy ions inthis object. Following similar analyses of other PNe we find that IC 418 showsa small discrepancy in ion abundances derived from forbidden vs. recombinationlines of the heavy elements.Comment: 24 pages, 10 figures, accepted by ApJ. Preprint utilizes emulateapj.cls v. 4/12/04 (included