2001A&A...370....1S

We constructed diagnostic diagrams using emission line ratios and equivalent widths observed in several independent samples of H II galaxies. Significant trends are seen, both in the line ratio diagrams, and in diagrams relating line ratios to the equivalent width of Hβ. The diagrams are compared to predictions from photoionization models for evolving starbursts. This study extends the work of Stasinska & Leitherer (1996ApJS..107..661S) by including objects with no direct determination of the metallicities, and by using updated synthesis models with more recent stellar tracks and atmospheres. We find that H II galaxies from objective-prism surveys are not satisfactorily reproduced by simple models of instantaneous starbursts surrounded by constant density, ionization bounded H II regions. The observed relations between emission line ratios and Hβ equivalent width can be understood if older stellar populations generally contribute to the observed optical continuum in H II galaxies. In addition, different dust obscuration for stars and gas and leakage of Lyman continuum photons from the observed H II regions can be important. As a result, H II galaxies selected from objective-prism surveys are not likely to contain significant numbers of objects in which the most recent starburst is older than about 5Myr. This explains the success of the strong line method to derive oxygen abundances, at least in metal poor H II galaxies. The observed increase of [O I]/Hβ with decreasing Hβ equivalent width can result from the dynamical effects of winds and supernovae. This interpretation provides at the same time a natural explanation of the small range of ionization parameters in giant H II regions. The classical diagnostic diagram [O III]/Hβ vs. [O II]/Hβ cannot be fully understood in terms of pure photoionization models. The largest observed [O II]/Hβ ratios require additional heating. The [N II]/[O II] ratio is shown to increase as the Hβ equivalent width decreases. A possible explanation is an N/O increase due to gradual enrichment by winds from Wolf-Rayet stars on a time scale of ∼5 Myr. Alternatively, the relation between N/O and O/H could be steeper than N/O ∝ O/H^0.5, with a previous stellar generation more important at higher metallicities.