An Empirical Test and Calibration of HiiRegion Diagnostics

We present spectrophotometry in the 3600-9700 A region for a sample of 39 HII regions in the Galaxy and Magellanic Clouds, for which independentinformation is available on the spectral types and effective temperatures ofthe ionizing stars. The spectra have been used to evaluate nebular diagnosticsof stellar temperature, metal abundance, and ionization parameter, and comparethe observed behavior of the line indices with predictions of nebularphotoionization models. We observe a strong degeneracy between forbidden-linesequences produced by changes in stellar Teff and metal abundance, whichseverely complicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds and Pagel[O II]+[O III] abundance index and the Vilchez and Pagel `eta' index providemore robust diagnostics of metal abundance and stellar effective temperature,respectively. A comparison of the fractional helium ionization of the H IIregions with stellar temperature confirms the reliability of the spectral typevs Teff calibration for the relevant temperature range Teff < 38000 K. We useempirical relations between the nebular hardness indices and Teff toreinvestigate the case for systematic variations in the stellar effectivetemperatures and the upper IMFs of massive stars in extragalactic H II regions.The data are consistent with a significant softening of the ionizing spectra(consistent with cooler stellar temperatures) with increasing metal abundance,especially for Z less than solar. However unresolved degeneracies between Z andTeff still complicate the interpretation of this result.Comment: 28 pages, 14 figures, 2 tables. To appear in The Astrophysical Journa