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 a bundances of the most common chemical elements. The range of O/H spann ed by the data is from 0.5 to 0.03 the solar value. The rarity of obje cts in the sample with metallicities similar to that of I Zw 18 could correspond to the claimed increasing probability to find low-metallici ty objects among intrinsically fainter systems. It is argued that the correction for ionization for Ne could be inadequate due to the transf ormation of part of the O++ into O+ in the high-ionization zone throug h charge transfer reactions. The apparent overabundance of Ne found in low-excitation objects would then be just an artifact. When the analy sis is restricted to high-excitation nebulae (O++/O greater than or eq ual 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 shoc ks 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 ha nd, the greater N abundances in higher metallicity systems would indic ate that secondary production is at work in them. Those mechanisms, ho wever, are operative only when the metallicity is greater than some th reshold value. The last element we have considered is helium. It is ar gued that none of the proposed ionization correction schemes works cor rectly, so only objects for which the amount of neutral He is seemingl y negligible can be retained to determine He abundances. It is remarke d 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.