The evolution of Galactic planetary nebula progenitors through the
  comparison of their nebular abundances with AGB yields
  We study the chemical abundances of a wide sample of 142 Galactic planetary
nebulae (PNe) with good quality observations, for which the abundances have
been derived more or less homogeneously, thus allowing a reasonable comparison
with stellar models. The goal is the determination of mass, chemical
composition and formation epoch of their progenitors, through comparison of the
data with results from AGB evolution. The dust properties of PNe, when
available, were also used to further support our interpretation.
  We find that the majority ($\sim60\%$) of the Galactic PNe studied has nearly
solar chemical composition, while $\sim40\%$ of the sources investigated have
sub-solar metallicities. About half of the PNe have carbon star progenitors, in
the $1.5~M_{\odot} < M < 3~M_{\odot}$ mass range, which have formed between 300
Myr and 2 Gyr ago. The remaining PNe are almost equally distributed among PNe
enriched in nitrogen, which we interpret as the progeny of $M > 3.5~M_{\odot}$
stars, younger than 250 Myr, and a group of oxygen-rich PNe, descending from
old ($> 2$ Gyr) low-mass ($M < 1.5~M_{\odot}$) stars that never became C-stars.
  This analysis confirms the existence of an upper limit to the amount of
carbon which can be accumulated at the surface of carbon stars, probably due to
the acceleration of mass loss in the late AGB phases. The chemical composition
of the present sample suggests that in massive AGB stars of solar (or slightly
sub-solar) metallicity, the effects of third dredge up combine with hot bottom
burning, resulting in nitrogen-rich - but not severely carbon depleted -
gaseous material to be ejected.
