Abundances of Galactic Anticenter Planetary Nebulae and the Oxygen
  Abundance Gradient in the Galactic Disk
  We have obtained spectrophotometric observations of 41 anticenter planetary
nebulae (PNe) located in the disk of the Milky Way. Electron temperatures and
densities, as well as chemical abundances for He, N, O, Ne, S, Cl, and Ar were
determined. Incorporating these results into our existing database of PN
abundances yielded a sample of 124 well-observed objects with
homogeneously-determined abundances extending from 0.9-21 kpc in galactocentric
distance. We performed a detailed regression analysis which accounted for
uncertainties in both oxygen abundances and radial distances in order to
establish the metallicity gradient across the disk to be:
12+log(O/H)=(9.09+/-.05) - (0.058+/-.006) x Rg, with Rg in kpc. While we see
some evidence that the gradient steepens at large galactocentric distances,
more objects toward the anticenter need to be observed in order to confidently
establish the true form of the metallicity gradient. We find no compelling
evidence that the gradient differs between Peimbert Types I and II, nor is
oxygen abundance related to the vertical distance from the galactic plane. Our
gradient agrees well with analogous results for H II regions but is steeper
than the one recently published by Stanghellini & Haywood (2010) over a similar
range in galactocentric distance. A second analysis using PN distances from a
different source implied a flatter gradient, and we suggest that we have
reached a confusion limit which can only be resolved with greatly improved
distance measurements and an understanding of the natural scatter in oxygen
abundances. Finally, a consideration of recently published chemical evolution
models of the Galactic disk suggests that reconciling the current range in
published oxygen gradients is necessary for adequately constraining parameters
such as the surface density threshold for star formation and the characteristic
timescale for disk formation.
