An empirical test and calibration of HII region diagnostics

We present spectrophotometry in the 3600-9700 Angstrom region for a sample of 39 H II regions in the Galaxy and Magellanic Clouds, for which independe nt information is available on the spectral types and effective temperature s of the ionizing stars. The spectra have been used to evaluate nebular dia gnostics 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 f orbidden-line sequences produced by changes in stellar T-eff and metal abun dance, which severely complicates the application of many forbidden-line di agnostics to extragalactic H II regions. Our data confirm however that the Edmunds & Pagel [O II] + [O III] abundance index and the Vilchez & Pagel et a' index provide more robust diagnostics of metal abundance and stellar eff ective temperature, respectively. A comparison of the fractional helium ion ization of the H II regions with stellar temperature confirms the reliabili ty of the spectral type versus T-eff calibration for the relevant temperatu re range T-eff less than or equal to 38,000 K. We use empirical relations b etween the nebular hardness indices and T-eff to reinvestigate the case for systematic variations in the stellar effective temperatures and the upper initial mass functions of massive stars in extragalactic H II regions. The data are consistent with a significant softening of the ionizing spectra (c onsistent with cooler stellar temperatures) with increasing metal abundance , especially for Z less than or equal to Z.. However, unresolved degeneraci es between Z and T-eff still complicate the interpretation of this result.