ABUNDANCES OF H-II REGIONS IN EARLY-TYPE SPIRAL GALAXIES

We present an abundance study of H II regions in 15 Sa to Sb galaxies. We find that the metal abundances of the H II regions are systematica lly higher than those in Sc and later type galaxies, although this eff ect is not a function of Hubble type only. The distribution of reddeni ng values is similar to late-type spirals, indicating that the dust le vels associated with our sample of H II regions are not unusually high . This implies that the systematic changes in H II region luminosity a long the Hubble sequence are due to intrinsic differences in the prope rties of the gas and ionizing OB associations, rather than to differen ces in extinction. Our data also suggest that the abundance gradients in early-type galaxies may generally be flatter than in Sc galaxies. T he metallicities within our sample exhibit no correlation with galaxy mass, suggesting that mass is not the only factor driving the observed gas-phase abundances among luminous (M(B) < -20) spiral galaxies. We have also computed photoionization models of metal-rich (0.5-3 Z.) H I I regions, in order to evaluate the reliability of the Pagel & Edmunds R23 empirical abundance parameter in this abundance range. The result s indicate that R23 provides a valuable means of ranking relative abun dances in this regime, but systematic variations in other parameters, most notably nebular density and stellar effective temperature, introd uce significant uncertainties into the abundance scales.