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Radial abundance gradients are a common feature of spiral galaxies, and inthe case of the Galaxy both the magnitude of the gradients and their variationsare among the most important constraints of chemical evolution models.Planetary nebulae (PN) are particularly interesting objects to study thegradients and their variations. Owing to their bright emission spectra, theycan be observed even at large galactocentric distances, and the derivedabundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex,particularly for the elements that are not synthesized in their progenitorstars. On the other hand, as the offspring of intermediate mass stars, withmain sequence masses in the interval of 1 to 8 solar masses, they arerepresentative of objects with a reasonable age span. In this paper, we presentan estimate of the time variation of the O/H radial gradient in a samplecontaining over 200 nebulae with accurate abundances. Our results areconsistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to-0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpcduring the last 5 Gyr.

An estimate of the time variation of the O/H radial gradient from planetary nebulae