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

  • Sharpee B
  • Baldwin J
  • Williams R
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Abstract

We present high signal-to-noise ratio echelle spectra of the compact high surface brightness, low-ionization planetary nebula (PN) IC 418. These reveal 807 emission lines down to intensities less than 10$^{-5}$ that of H{$β$} 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 versus recombination lines of the heavy elements.

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Sharpee, B., Baldwin, J. A., & Williams, R. (2004). Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418. The Astrophysical Journal, 615(1), 323–343. https://doi.org/10.1086/424035

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