Precision abundance analysis of bright HII galaxies

We present high signal-to-noise ratio spectrophotometric observations of seven luminous H II galaxies. The observations have been made with the use of a double-arm spectrograph which provides spectra with a wide wavelength coverage, from 3400 to 10 400 angstrom free of second-order effects, of exactly the same region as that of a given galaxy. These observations are analysed applying a methodology designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the ionized gas. Four electron temperatures and one electron density are derived from the observed forbidden line ratios using the five-level atom approximation. For our best objects, errors of 1 per cent in t(e)([O III]), 3 per cent in t(e)([O II]) and 5 per cent in t(e)([S III]) are achieved with a resulting accuracy of 7 per cent in total oxygen abundances, O/H. The ionization structure of the nebulae can be mapped by the theoretical oxygen and sulphur ionic ratios, on the one side, and the corresponding observed emission line ratios, on the other - the eta and eta' plots. The combination of both is shown to provide a means to test photoionization model sequences presently applied to derive elemental abundances in H II galaxies.