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We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact HIIregions in the nearby spiral galaxy NGC 300, and analyzed them together withthose of the giant HII regions previously observed. We have determined thephysical properties of all these objects and their He, N, O, Ne, S and Arabundances in a consistent way.  We find that, globally, compact HII regions have abundance ratios similar tothose of giant HII regions, while PNe have systematically larger N/O ratios andsimilar Ne/O and Ar/O ratios. We demonstrate that the nitrogen enhancement inPNe cannot be only due to second dredge-up in the progenitor stars, since theirinitial masses are around 2--2.5\,\msun. An extra mixing process is required,perhaps driven by stellar rotation.  Concerning the radial abundance distribution, PNe behave differently from HIIregions: in the central part of the galaxy their average O/H abundance ratio is0.15 dex smaller. Their abundance dispersion at any galactocentric radius issignificantly larger than that shown by HII regions and many of them have O/Hvalues higher than HII regions at the same galactocentric distance. Thissuggests that not only nitrogen, but also oxygen is affected by nucleosynthesisin the PN progenitors, by an amount which depends at least on the stellarrotation velocity and possibly other parameters. The formal O/H, Ne/H and Ar/Heabundance gradients from PNe are significantly shallower that from HII regions.We argue that this indicates a steepening of the metallicity gradient in NGC300 during the last Gyr, rather than an effect of radial stellar motions,although the large observed dispersion makes this conclusion only tentative.

Planetary nebulae and HII regions in the spiral galaxy NGC 300. Clues on the evolution of abundance gradients and on AGB nucleosynthesis