Discrepancies between the [O iii] and [S iii] temperatures in H ii regions

Context. Analysis of published [O iii] and [S iii] temperatures measurements of emission line objects consisting of Hii galaxies, giant extragalactic Hii regions, Galactic Hii regions, and Hii regions from the Magellanic Clouds reveal that the [O iii] temperatures are higher than the corresponding values from [S iii] in most objects with gas metallicities in excess of 0.2 solar. For the coolest nebulae (the highest metallicities), the [O iii] temperature excess can reach ~3000 K. Aims. We look for an explanation for these temperature differences and explore the parameter space of models with the aim of reproducing the observed trend of TO iii > TS iii in Hii regions with temperatures below 14 000 K. Methods. Using standard photoionization models, we varied the ionization parameter, the hardness of the ionizing continuum, and the gas metallicities in order to characterize how models behave with respect to the observations. We introduced temperature inhomogeneities and varied their mean squared amplitude t2 . We explored the possibility of inhomogeneities in abundances by combining two models of widely different metallicity. We calculated models that consider the possibility of a non-Maxwell-Boltzmann energy distribution (a κ-distribution) for the electron energies. We also considered shock heating within the photoionized nebula. Results. Simple photoionization calculations yield nearly equal [O iii] and [S iii] temperatures in the domain of interest. Hence these models fail to reproduce the [O iii] temperature excess. Models that consider temperature inhomogeneities, as measured by the mean squared amplitude t2 , also fail in the regime where TO iii