ELEMENT ABUNDANCES IN H-II GALAXIES

Data for 121 H II systems from the spectrophotometric catalog of H II galaxies (Terlevich et al. 1991) have been analyzed to determine the abundances of the most common chemical elements. The range of O/H spanned by the data is from 0.5 to 0.03 the solar value. The rarity of objects in the sample with metallicities similar to that of I Zw 18 could correspond to the claimed increasing probability to find low-metallicity objects among intrinsically fainter systems. It is argued that the correction for ionization for Ne could be inadequate due to the transformation of part of the O++ into O+ in the high-ionization zone through charge transfer reactions. The apparent overabundance of Ne found in low-excitation objects would then be just an artifact. When the analysis is restricted to high-excitation nebulae (O++/O greater than or equal to 0.9), we find a constant Ne/O=0.17 ratio, in agreement with the current ideas about the nucleosynthesis of those two elements. The N/O ratio, once the systems with detected traces of the presence of shocks were discarded, was found to be constant at the level 0.032. Thus, for those objects, N is essentially primary in origin. On the other hand, the greater N abundances in higher metallicity systems would indicate that secondary production is at work in them. Those mechanisms, however, are operative only when the metallicity is greater than some threshold value. The last element we have considered is helium. It is argued that none of the proposed ionization correction schemes works correctly, so only objects for which the amount of neutral He is seemingly negligible can be retained to determine He abundances. It is remarked that the selection by effective temperature corresponds in fact to a selection by metallicity. The lower envelope of the He abundances we find is about 22%. We do not find any trend between the He content and the abundances of other elements like O or N.