Physical Conditions of the Small Magellanic Cloud HII Region NGC 456 and the Underestimated Heavy Element Abundances of the Universe

We study the O/H abundance determinations in HII regions in the presence of temperature inhomogeneities (as proposed by Peimbert in 1967), t². The first object of our study is NGC 456, which the second brightest HII region of the Small Magellanic Clouds (SMC); we find it has an abundance 1.75 times higher than previously determined. Traditionally it is assumed that these objects have the same temperature throughout the whole volume; this temperature is then used to calculate abundances. Our determination was done using Collisionally Excited Lines (CEL) and the t² obtained from HeI lines; it is consistent with abundances determined with Recombination Lines (RL). The most well known strong-line ratio or metallicity indicator used to determine the O/H ratio is O₂₃= ([OII]3727Å + [OIII]4959,5007Å) / Hβ introduced by Pagel et al. (1979). The log(O₂₃) vs. 12+log(O/H) diagram constitutes an important tool in the study of objects with low intrinsecal brightness or high redshift. To account for the presence of thermal inhomogeneities, the upper branch of the O₂₃-O/H diagram can be recalibrated using RLs whereas in the lower branch of the diagram RLs cannot be used due to their faintness. For low metallicity objects the formalism of t² is applied to CELs to determine abundances and recalibrate the O₂₃-O/H diagram.

We find that the point corresponding to NGC 456 in the diagram shifts up by 0.24 dex in the 12+log(O/H) axis. This is consistent with preliminary results for the next 3 objects of our sample and with the behavior of other HII regions from the literature were t² has been measured. The systematic shift in the curve of the O₂₃-O/H diagram implies that abundances in the Universe need to be corrected by a factor of approximately 2.