8.5 Future Needs and Directions

At the present time, the most serious deficiency in the PNLF program is a lack of reliable external comparisons. Because the PNLF measurements to date have concentrated on elliptical and S0 galaxies, only three galaxies have both a PNLF and Cepheid distance determination: M31 (the PNLF calibrator galaxy), the LMC (which has a sparse PNLF, and thus a relatively large internal error), and M81 (whose Cepheid distance is based on only two objects with uncertain reddening). Although PNLF measurements in late type spirals are difficult, such measurements are critical for understanding possible systematic errors in the method.

Also needed for a better understanding of the PNLF are observations of PN in the light of other species. Until now, planetary nebula luminosity functions have only been measured in [O III] and H. However, one of the best ways of exploring the physics of bright planetaries is to perform observations in [N II] 6584 and C III] 1909. Both nitrogen and carbon are important coolants in the nebula, and their luminosity functions are predictable using the modeling techniques of Jacoby (1989). If the dredge-up of partially burned material indeed plays a role in the rapid 5007 cutoff, then this effect will be reflected in the PNLF of [N II] and C III] as well. For example, if an increase in nitrogen occurs at the expense of carbon in stars above a certain core mass, then this abundance change will not only cause an abrupt truncation in the C III] 1909 luminosity function, but will also help truncate the [O III] 5007, since nitrogen becomes very efficient at cooling these objects. The observations of the C III] 1909 luminosity function in the bulge of M31 will therefore be an important test of this scenario.