Chemical evolution and abundance gradients in spiral galaxies

Chemical evo-\ud lution models of spiral galaxies require observational constraints in order to make accurate predictions\ud concerning interstellar abundances, star formation history, age-metallicity relations, etc. Radial abun-\ud dance gradients constitute one of the most interesting constraints, due to the fact that several chemical\ud elements can be observed and, apart from the average gradients, spatial and temporal variations are also\ud observed. In other words, gradients are responsible for a series of observational constraints that must\ud be satisfied by any realistic chemical evolution model. In this work, we present new radial abundance\ud gradients for the Milky Way disk derived from different samples of galactic planetary nebulae. The\ud objects are divided into groups according to the age of the progenitor stars, so that we can obtain an\ud estimate of the time variation of the gradients since the oldest progenitors were born. Several methods\ud have been employed to estimate the stellar ages, based either on the nebular abundances or on their\ud kinematic properties. The derived results suggest very small or no variations during the last 3 to