Planetary nebulae in M33: probes of asymptotic giant branch nucleosynthesis and interstellar medium abundances☆

We have obtained deep optical spectrophotometry of 16 planetary nebulae (PNe) in M33, mostly located in the central 2 kpc of the galaxy, with the Subaru and Keck telescopes. We have derived electron temperatures and chemical abundances from the detection of the [O iii] lambda 4363 line for the whole sample. We have found one object with an extreme nitrogen abundance, 12 + log(N/H) = 9.20, accompanied by a large helium content. After combining our data with those available in the literature for PNe and H ii regions, we have examined the behaviour of nitrogen, neon, oxygen and argon in relation to each other, and as a function of galactocentric distance. We confirm the good correlation between Ne/H and O/H for the PNe in M33. Ar/H is also found to correlate with O/H. This strengthens the idea that at the metallicity of the bright PNe analysed in M33, which is similar to that found in the Large Magellanic Cloud, these elements have not been significantly modified during the dredge-up processes that take place during the asymptotic giant branch phase of their progenitor stars. We find no significant oxygen abundance offset between PNe and H ii regions at any given galactocentric distance, despite the fact that these objects represent different age groups in the evolution of the galaxy. Combining the results from PNe and H ii regions, we obtain a representative slope of the interstellar medium alpha element (O, Ar, Ne) abundance gradient in M33 of -0.025 +/- 0.006 dex kpc-1. Both PNe and H ii regions display a large abundance dispersion at any given distance from the galactic centre. We find that the N/O ratio in the PNe is enhanced, relative to that in the H ii regions, by approximately 0.8 dex.