Spectroscopic observations of planetary nebulae are presently the only way to measure the chemical abundances of individual elements in old and intermediate age stars at distances of ~ 1 Mpc or greater. Although the nebular abundances of He, C, N, and S are enhanced by nucleosynthesis in the parent star (e.g. Dopita et al. 1997), comparisons can be made of relative abundances from one part of a galaxy to another, and between galaxies.
In the LMC, [O/H] in bright PNe has the same value as [O/H] in HII regions (Richer 1993); consequently, the oxygen abundance measured in PNe is a good measure of the oxygen abundance in the gas from which the parent stars formed.
With the exception of the He/H ratio, direct abundance determinations
require the measurement of the electron temperature
Te, the electron
density ne, and line intensities in two or more ionization
stages. Because the crucial temperature diagnostic line [OIII]
4363 is 50 to 200
times weaker than [OIII]
5007, the
measurement of this faint line sets the distance limit for abundance
determinations. To date, direct abundance measurements have not been
made in galaxies more distant than M31. Indirect PNe abundance
determinations based on diagnostic line ratios have been made in
NGC 5128
(Walsh et al. 1999).
Jacoby & Ciardullo (1999) measured the chemical abundances in 15 M31 PNe. Their paper, which finds a wide range in [O/H] in M31's old stellar populations, is a model for these difficult observations.