Independent Emission and Absorption Abundances for Planetary Nebulae

TL;DR

This study compares UV absorption line data with optical emission line data in planetary nebulae to address longstanding discrepancies in abundance measurements, finding better agreement with forbidden lines than recombination lines.

Contribution

It introduces a method of using UV resonance line absorption to independently measure ion abundances in planetary nebulae, clarifying the reliability of emission line diagnostics.

Findings

Forbidden line abundances agree with UV absorption measurements.

Recombination line abundances show poorer agreement, near uncertainty limits.

Further studies needed with larger samples to confirm results.

Abstract

Emission-line abundances have been uncertain for more than a decade due to unexplained discrepancies in the relative intensities of the forbidden lines and weak permitted recombination lines in planetary nebulae (PNe) and H II regions. The observed intensities of forbidden and recombination lines originating from the same parent ion differ from their theoretical values by factors of more than an order of magnitude in some of these nebulae. In this study we observe UV resonance line absorption in the central stars of PNe produced by the nebular gas, and from the same ions that emit optical forbidden lines. We then compare the derived absorption column densities with the emission measures determined from ground-based observations of the nebular forbidden lines. We find for our sample of PNe that the collisionally excited forbidden lines yield column densities that are in basic agreement with the column densities derived for the same ions from the UV absorption lines. A similar comparison involving recombination line column densities produces poorer agreement, although near the limits of the formal uncertainties of the analyses. An additional sample of objects with larger abundance discrepancy factors will need to be studied before a stronger statement can be made that recombination line abundances are not correct.