An Empirical Test and Calibration of H ii Region Diagnostics

Abstract

We present spectrophotometry in the 3600È9700 region for a sample of 39 H II regions in the A Ž Galaxy and Magellanic Clouds, for which independent information is available on the spectral types and e †ective temperatures of the ionizing stars. The spectra have been used to evaluate nebular diagnostics of stellar temperature, metal abundance, and ionization parameter, and to compare the observed behavior of the line indices with predictions of nebular photoionization models. We observe a strong degeneracy between forbidden-line sequences produced by changes in stellar and metal abundance, which severe-T eff ly complicates the application of many forbidden-line diagnostics to extragalactic H II regions. Our data conÐrm however that the Edmunds & Pagel [O II] ] [O III] abundance index and the & Pagel V• lchez g@ index provide more robust diagnostics of metal abundance and stellar e †ective temperature, respectively. A comparison of the fractional helium ionization of the H II regions with stellar temperature con-Ðrms the reliability of the spectral type versus calibration for the relevant temperature range T eff T eff ¹ 38,000 K. We use empirical relations between the nebular hardness indices and to reinvestigate the T eff case for systematic variations in the stellar e †ective temperatures and the upper initial mass functions of massive stars in extragalactic H II regions. The data are consistent with a signiÐcant softening of the ionizing spectra (consistent with cooler stellar temperatures) with increasing metal abundance, especially for However, unresolved degeneracies between Z and still complicate the interpretation of Z ¹ Z _. T eff this result.