The Origin of the Consistent Planetary Nebula Luminosity Function Bright-end Cutoff

TL;DR

This paper investigates why the brightest planetary nebulae have a consistent luminosity cutoff across different galaxy types, using simulations to identify key physical factors influencing this uniformity.

Contribution

It reveals that the [O III] luminosity cutoff depends on dust-to-gas ratio and stellar luminosity, providing a physical explanation for the PNLF uniformity and predicting testable observational signatures.

Findings

Peak [O III] luminosity depends weakly on stellar temperature and ejecta mass.

Peak [O III] luminosity depends strongly on stellar luminosity and dust-to-gas ratio.

White dwarf mergers may produce nebulae contributing to the bright-end cutoff.

Abstract

The [O III] 5007 Angstrom line is typically the brightest line in planetary nebula (PN) spectra. Observations show that the brightest [O III] 5007 Angstrom PN in a galaxy -- the planetary nebula luminosity function (PNLF) bright-end cutoff -- is surprisingly independent of galaxy type. To understand the origin of this puzzling uniformity, we simulate PNe with a range of cloud and star parameters using the photoionization code CLOUDY. We find that the peak [O III] 5007 Angstrom luminosity depends weakly on both the central stellar effective temperature at high temperature and on the total PN ejecta mass; however, the peak [O III] 5007 Angstrom luminosity depends strongly on the central stellar luminosity and the PN dust-to-gas mass ratio. We explain these scalings physically. They imply that a higher dust-to-gas mass ratio at higher central stellar luminosity can help explain a constant bright-end cutoff in the PNLF across galaxy types. This prediction is testable with a survey of galactic PNe. The surviving remnants of double white dwarf mergers should also produce photoionized nebulae analogous to PNe. These may be preferentially present at the high luminosity end of the [O III] PLNF and could explain the existence of PNe in early-type galaxies that are more luminous in [O III] than expected from single-star evolutionary models. The presence of white dwarf mergers in both young and old stellar populations could contribute to the uniformity of the [O III] PNLF across galaxy types; such nebulae would lack the hydrogen lines otherwise characteristic of PNe.