Chemical abundances of planetary nebulae in the bulge and disk of M31

We derive abundances and central star parameters for 15 planetary nebulae (PNe) in M31: 12 in the bulge and three in a disk field 14 kpc from the nucleus. No single abundance value characterizes the bulge stars: although the median abundances of the sample are similar to those seen for PNe in the LMC, the distribution of abundances is several times broader, spanning over one decade. None of the PNe in our sample approach the super-metal-rich ([Fe/H] ∼ 0.25) expectations for the bulge of M31, although a few PNe in the sample of Stasińska, Richer, & Mc Call come close. This [O/H] versus [Fe/H] discrepancy is likely due to a combination of factors, including an inability of metal-rich stars to produce bright PNe, a luminosity selection effect, and an abundance gradient in the bulge of M31. We show that PNe that are near the bright limit of the [O III] λ5007 planetary nebula luminosity function (PNLF) span nearly a decade in oxygen abundance and, thus, support the use of the PNLF for deriving distances to galaxies (in the work by Jacobi and collaborators) with differing metallicities. We also identify a correlation between central star mass and PN dust formation that partially alleviates any dependence of the PNLF maximum magnitude on population age. In addition, we identify a spatially compact group of five PNe having unusually high O/H; this subgroup may arise from a recent merger, but velocity information is needed to assess the true nature of the objects.