The properties of bright planetary nebulae in the Magellanic Clouds and the Milky Way were investigated to determine whether bright planetary nebulae are suitable as oxygen abundance probes for chemical evolution studies, particularly in galaxies without current star formation. It was found that bright planetary nebulae do not modify their initial oxygen abundance, that the central star's ultraviolet luminosity is the most important factor affecting the nebular (O scIII) lambda5007 luminosity, and that the maximum (O scIII) lambda5007 luminosity attained increases with increasing oxygen abundance. Consequently, a luminosity-based selection will preferentially select oxygen-rich planetary nebulae, a bias that makes bright planetary nebulae particularly suitable probes for chemical evolution studies in star-forming galaxies. Theoretical models were used to investigate whether this circumstance persists in galaxies where star formation stopped long ago. These models predict luminosity functions for all galaxies and abundance distributions for the Magellanic Clouds that are similar to those observed. In all galaxies, these models predict that a gap develops between the abundances observed in bright planetary nebulae and those that persisted in the interstellar medium when star formation stopped. This abundance gap is a function of the oxygen abundance achieved in the interstellar medium when star formation stopped, rather than of the model details, and is always less than 0.5 dex. For the Milky Way, the predicted abundance gap, 0.14 dex, is identical to that observed. Oxygen abundances for planetary nebulae were used. to investigate whether diffuse elliptical and dwarf irregular galaxies are related by evolution. Diffuse ellipticals are found to have larger oxygen abundances than similarly luminous dwarf irregulars, and to have larger (O/Fe) ratios than dwarf irregulars with the same oxygen abundance. The simplest explanation for both of these observations is that diffuse ellipticals formed on shorter time scales than dwarf irregulars. Given their different star formation histories, diffuse ellipticals cannot be the faded remnants of dwarf irregulars. Since extragalactic planetary nebulae are faint, two methods were developed to determine empirical oxygen abundances. These methods are based upon the observations that the maximum (O scIII) lambda5007 luminosity and the maximum (O scIII) lambda5007/Hβ ratio both increase with increasing oxygen abundance.
- Pub Date:
- January 1994
- OXYGEN ABUNDANCE;
- CHEMICAL EVOLUTION;
- Physics: Astronomy and Astrophysics