BRIGHT PLANETARY-NEBULAE AS PROBES OF CHEMICAL EVOLUTION

A compilation of observations of planetary nebulae in the Magellanic Clouds has been used to study the utility of bright extragalactic planetary nebulae as distance indicators and as probes of chemical evolution. A planetary nebula's [O III] lambda5007 luminosity is shown to be primarily a result of the ultraviolet luminosity of its central star, confirming Jacoby's (1989) original proposal. However, the oxygen abundance is observed to modify the maximum [O III] lambda5007 luminosity actually attained. As a result, the planetary nebula luminosity function will overestimate the distances to low-luminosity galaxies. A procedure to correct the distance moduli of low-luminosity galaxies relative to the LMC is outlined. As chemical abundance probes, the brightest planetary nebulae in a galactic population appear to be very promising. A luminosity-limited study will preferentially select the planetary nebulae with the highest oxygen abundances because of the abundance dependence of the [O III] lambda5007 luminosity. In the Magellanic Clouds, H II regions and bright planetary nebulae have similar oxygen abundances, so, from a chemical evolution viewpoint, both must represent recent star formation events. Consequently, bright planetary nebulae should be reliable probes of the interstellar medium oxygen abundance during the last episode of star formation, which is especially useful in galaxies that are no longer forming stars. If the width of the SMCs abundance distribution is typical of that in low-luminosity galaxies, only three planetary nebulae are required to determine the oxygen abundance at the last epoch of star formation to within +/-0.1 dex.