THE GALACTIC STRUCTURE AND CHEMICAL EVOLUTION TRACED BY THE POPULATION OF PLANETARY NEBULAE

Planetary nebulae (PNe) derive from the evolution of similar to 1-8 M(circle dot) mass stars, corresponding to a wide range of progenitor ages, and thus are essential probes of the chemical evolution of galaxies, and indispensable to constrain the results from chemical models. We use an extended and homogeneous data set of Galactic PNe to study the metallicity gradients and the Galactic structure and evolution. The most up-to-date abundances, distances (calibrated with Magellanic Cloud PNe), and other parameters have been employed, together with a novel homogeneous morphological classification, to characterize the different PN populations. We confirm that morphological classes have a strong correlation with Peimbert's type PN, and also with their distribution on the Galactic landscape. We studied the a-element distribution within the Galactic disk, and found that the best selected disk population (i.e., excluding bulge and halo component), together with the most reliable PN distance scale yields to a radial oxygen gradient of Delta log(O/H)/Delta R(G) = -0.023 +/- 0.006 dex kpc(-1) for the whole disk sample, and of Delta log(O/H)/Delta R(G) = -0.035 +/- 0.024, -0.023 +/- 0.005, and -0.011 +/- 0.013 dex kpc(-1), respectively for Type I, II, and III PNe, i.e., for high-, intermediate-, and low-mass progenitors. Neon gradients for the same PN types confirm the trend. Accurate statistical analysis shows moderately high uncertainties in the slopes, but also confirms the trend of steeper gradient for PNe with more massive progenitors, indicating a possible steepening with time of the Galactic disk metallicity gradient for what the alpha-elements are concerned. We found that the metallicity gradients are almost independent on the distance scale model used, as long as these scales are equally well calibrated with the Magellanic Clouds. The PN metallicity gradients presented here are consistent with the local metallicity distribution; furthermore, oxygen gradients determined with young and intermediate age PNe show good consistency with oxygen gradients derived respectively from other young (OB stars, H II regions) and intermediate (open cluster) Galactic populations. We also extend the Galactic metallicity gradient comparison by revisiting the open cluster [Fe/H] data from high resolution spectroscopy. The analysis suggests that they could be compliant with the same general picture of a steepening of gradient with time.