Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion   Emission Densities for IC 418
  Recent analyses of nebular spectra have resulted in discrepant abundances from CNO forbidden and recombination lines. We consider independent methods of determining ion abundances for emission nebulae, comparing ion emission measures with column densities derived from resonance absorption lines viewed against the central star continuum. Separate analyses of the nebular emission lines and the stellar UV absorption lines yield independent abundances for ions, and their ratio can be expressed in terms of a parameter n_e_{em}, the ``emission density'' for each ion. Adequate data for this technique are still scarce, but separate analyses of spectra of the planetary nebula and central star of IC 418 do show discrepant abundances for several ions, especially Fe II. The discrepancies are probably due to the presence of absorbing gas which does not emit and/or to uncertain atomic data and excitation processes, and they demonstrate the importance of applying the technique of combining emission- and absorption-line data in deriving abundances for nebulae. 
Robert Williams (1), Edward B. Jenkins (2), Jack A. Baldwin (3), Brian   Sharpee (3) ((1) Space Telescope Science Institute,...