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ABUNDANCE GRADIENTS IN THE OUTER GALACTIC DISK FROM PLANETARY NEBULAE

Radial abundance gradients of the element ratios O/H, Ne/H, S/H, and Ar/H are determined on the basis of a sample of disk planetary nebulae. The behaviour of the gradi- ents at large distances from the galactic centre, R>R 0 =7 :6 kpc, is emphasized. It is concluded that the derived gradients are consistent with an approximately constant slope in the inner parts of the Galaxy, and some flattening for distances larger than R0. A comparison is made with previous determinations using both photoionized nebulae and young stars, and some conse- quences on theoretical models for the chemical evolution of the galactic disk are discussed.