2.2 Planetary Nebulae
Planetary nebulae (PNe) enable a spectroscopic abundance determination, using the same technique as for H II regions. The intrinsic uncertainty due to temperature fluctuations (Peimbert 1967) and gradients is probably at least 0.1 dex (Pagel 1997). The derived oxygen abundances reflect the initial composition of the star since the ejected gas that forms the PN has not been enriched in oxygen by the central nucleosynthesis (however see remark below) and that moreover, the hot ejecta has not mixed with the surrounding colder ISM. The progenitor stars of PNe have masses up to several times that of the Sun, meaning that the derived O abundance traces the ISM abundance 0.1 to several Gyr ago, depending on the mass of the progenitor. This is a large range in lookback time over which significant chemical evolution may have occurred, and thus it is important to assess the mass of the progenitor star. A model that relates the oxygen abundance as derived from PNe and H II regions (i.e. the present ISM abundance) has been presented by Richer et al. (1997). They find that the abundance gap, the difference between these two abundance estimators, is a function of the star formation history and metallicity. The more extended the epoch of star formation has been and the higher the ISM metallicity, the larger the abundance gap.
While the PNe abundances of oxygen traces the ISM abundance at the birth of the progenitor star, this is not true for carbon and nitrogen which have been enriched during the stellar evolution (Pagel 1997, p. 199). Moreover for some types (PNe type I, cf. Peimbert and Serrano 1980) there might have been also enrichment of oxygen.