We have performed a new empirical calibration of nebular abundances using the sulphur abundance parameter S23. This calibration is an alternative to the commonly used one based on the strong optical oxygen lines and presents several advantages. From the observational point of view, the lines are easily observable, both in low and high metallicity regions, and less affected by reddening. Furthermore, their intensities can be measured relative to nearby hydrogen recombination lines thus minimizing any effects due to uncertainties in flux calibration. On the theoretical side, their contribution to the cooling of the nebula becomes important at electron temperatures lower than in the case of the traditional O23 (previously called R23) and therefore its relation with oxygen abundance remains single-valued up to metallicities close to solar. Also, the fact that S23 is less dependent than O23 on ionization parameter reduces the scatter in the relation.
The application of this new metallicity calibration can provide more accurate abundance determinations for objects with logO23 between 0.5 and 1.2, i. e. oxygen abundances between 12 + log(O/H) = 7.20 ( 0.02 times solar) and 12 + log(O/H) = 8.80 ( 0.75 times solar). This is the range of metallicities found in HII galaxies and HII regions in irregular galaxies and outer galactic discs.
Regarding HII regions of higher metallicity, the composed parameter S23 / O23 can provide a better quantification of abundance gradients through galactic discs and might also hold the key to a future abundance calibration in this lower temperature regime.
This work has been partially supported by DGICYT project PB-96-052. We would like to thank Elena Terlevich and Bernard Pagel for a careful reading of the manuscript.