Planetary Nebulae as a Chemical Evolution Tool: Abundance Gradients

Abstract

We have studied the time variation of the radial abundance gradients using samples of planetary nebulae, open clusters, cepheids and other young objects. Based on the analysis of O/H and S/H abundances for planetary nebulae and metallicities of the remaining objects, we concluded that the gradients have been flattening out in the last 8 Gyr with an average rate of the order of 0.005 - 0.010 dex/(kpc Gyr). We have estimated the errors involved in the determination of the gradients, and concluded that the existence of systematic abundance variations is more likely than a simple statistical dispersion around a mean value.