The chemical gradient of oxygen in the Galaxy from planetary nebulae

The chemical gradient of oxygen in the Galaxy from planetary nebulae (PNe) has been closely investigated using a recent extensive fully homogenous set of chemical determinations (up to end of 2001) and various distant scales. The found gradient is quite flatter than believed so far, amounting to about -0.016 +/- 0.008 log(O/H) kpc(-1) insensitive to the distance scales, when all PNe of types I, II, III of Peimbert are used. The value is -0.02 +/- 0.01 if only type II PNe are considered, thus reducing the ambiguity introduced by the mixing up of progenitors coming from a large spread in mass. A second control set made by the chemical oxygen abundances determined by all studies between 2002 and 2005, with a data base of some 200 objects, provides a similar result. Since previous determinations do have a spread in the slope of the galactocentric oxygen gradient from PNe between -0.07 and -0.01, further work is required to assess the matter. If our low value will be confirmed, the suggestion arises that type II PNe, due to mixing processes, might have lost their ability to be good tracers of oxygen abundance in the interstellar medium at the time of formation of the progenitors.