Effects of star cluster evolution on emission line ratios and the application to H II galaxies Including a calibration of the R23 vs. 12+log(O/H) relationship

Amodel of spectral evolution of star forming galaxies of various metallicity has been used to study the relation between the optical emission line ratio R23= ([OII]λ3727 +[OIII]λλ4959,5007)/Hβ and the nebular oxygen abundance. It is shown that the scatter in an empirical comparison sample could be due to various upper stellar mass limits or different slopes of the stellar initial mass function in these objects. However, it is more likely to be an effect of evolution of the underlying stellar population, i.e. the star forming knots observed are of different ages, or equally, the temperature of the ionizing radiation is different. This is particularly evident studying the evolution a fewmillion years after the star formation has ceased. These calculations show that the so called lower branch of the relation is much less sensitive to age than the turn-over region or the upper branch using an instantaneous burst of star formation. This is because massive stars, which dominate the light in a young stellar population, of low metallicity remain at higher effective temperature compared to a star of highermetallicity. In a region of active star formation the R23 vs. oxygen abundance relation is even more pronounced since it proves to be very insensitive to age. In the absence of a temperature-sensitive line for the oxygen abundance determination an empirical or semiempirical approach is normally utilized relying on a calibration of the R23 vs. oxygen abundance relation. In this analysis a new calibration of the upper branch, based on a fit of theoretical curves to empirical data of the lower branch, is attempted.