ABUNDANCE GRADIENTS FROM DISK PLANETARY-NEBULAE - O, NE, S, AND AR

A new determination of the radial abundance gradients of O/H, Ne/H, S/ H, and Ar/H is made for disk planetary nebulae, that is, those objects of Peimbert types I, II, and III. On the basis of a sample containing 200 nebulae, it can be concluded that these gradients are generally s imilar, and of the same order of magnitude as the O/H gradient display ed by galactic H II regions. Some distance-independent correlations co nfirm the accuracy of the abundances and support the interpretation of the gradients in terms of chemical evolution models. The time evoluti on of the abundance gradients in the Milky Way is investigated, and th eir variation with the types of PN are compared with predictions of ch emical evolution models.