2018ApJ...862...45S

We use planetary nebulae (PNe) as probes to determine the Galactic radial oxygen gradients and other abundance patterns. We select data homogeneously from recent data sets, including PNe at large Galactocentric distances. The radial oxygen gradient calculated for the general PN population, which probes the region from the Galactic center out to ∼28 kpc, is shallow, with slope ∼-0.02 dex kpc^–1, in agreement with previous findings. We looked for time evolution of the metallicity gradient using PNe with different age progenitors as metallicity probes. We identify PNe whose progenitor stars are younger than 1 Gyr (YPPNe) and those whose progenitor stars are older than 7.5 Gyr (OPPNe) based on the comparison between evolutionary yields and elemental abundances of the PNe. By studying OPPNe and YPPNe separately, we found that (i) the OPPNe oxygen gradient is shallower (∼-0.015 dex kpc^–1) than that derived from YPPNe (∼-0.027 dex kpc^–1); (ii) the OPPNe inner radial distribution of oxygen is compatible with no gradient to the radial extent of the thick disk population (∼10 kpc), similarly to what has been observed in thick disk stars; and (iii) PNe (especially OPPNe) indicate that significant gradient slope is limited to Galactocentric distances between ∼10 and ∼13.5 kpc, as observed for open clusters and field stars. Outside this range, the distribution is almost flat. We found that the radial oxygen gradient is steeper for a PN population closer to the Galactic disk, similar to what is observed in the general stellar population by the SEGUE survey. We use our novel population dating to compare our results with current chemical evolutionary models and gradients from other Galactic populations for insight on galaxy chemical evolution.