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 surface brightness, low ionization planetary nebula IC 418. These reveal 807 emission lines down to intensities less than 10$^{-5}$ that of H$\beta$ for which we determine widths and relative intensities. We show that line profiles are a valuable parameter for making line identifications and in constraining the excitation mechanism of the lines. We present evidence that indicates that many supposed high-level recombination lines may in fact be excited by a process other than recombination. We contend from the detection of dielectronic recombination lines that their relatively low intensities argue against their making a significant contribution to level populations of the heavy ions in this object. Following similar analyses of other PNe we find that IC 418 shows a small discrepancy in ion abundances derived from forbidden vs. recombination lines of the heavy elements. 
Brian Sharpee (1), Jack A. Baldwin (1), and Robert Williams (2)((1)   Michigan State University, (2) Space Telescope Science...