Abundances in M33 HII regions

These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12+log(O/H)=8.36+/-0.04 and a slope of -0.027+/-0.012dex/kpc, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements.

There is substantial intrinsic scatter of 0.11dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.