A comprehensive study of reported high-metallicity giant H II regions - I. Detailed abundance analysis

We present long-slit observations in the optical and near-infrared of 14 HII regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios are obtained and, for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of 12+log(O/H)=8.95+/-0.20, and constitutes, to the best of our knowledge, the first high-metallicity HII region for which accurate line temperatures, and hence elemental abundances, have been derived. For the rest of the regions no line temperature measurements could be made, and the metallicity has been determined by means of both detailed photoionization modelling and the sulphur abundance parameter S₂₃. Only one of these regions shows values of O₂₃ and S₂₃ implying a solar or oversolar metallicity. According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. These two fit the trends previously found in other high-metallicity HII regions, i.e., N/O and S/O abundance ratios seem to be higher and lower than solar respectively.