Some Implications of Galactic Abundance Gradients Derived from H II   Regions and Planetary Nebulae
  By comparing observed abundance gradients with those predicted by a chemodynamical model of the Galaxy the following results are obtained: there is a defficiency of O/H poor PNe in the solar vicinity which implies that only a fraction of intermediate mass stars produce PNe, a similar result is obtained from the observed abundance distributions of Ne/H, S/H and Ar/H; the age distributions of the stellar progenitors of type II and type III PNe show a substantial overlap, but the average age of the progenitors of type III PNe is larger than that of the progenitors of type II PNe; the S/H and the Ar/H gradients of type I PNe and H II regions are similar, the flatter O/H gradient of type I PNe relative to that of H II regions is probably to the effect of temperature fluctuations on the abundance determinations; the similarity of the abundance gradients derived from H II regions, and PNe imply that transient phenomena are not important in shaping the present day H II gradients. 
Manuel Peimbert & Leticia Carigi