Calibration of Nebular Emission‐Line Diagnostics. I. Stellar Effective Temperatures

We present a detailed comparison of optical H II region spectra tophotoionization models based on modern stellar atmosphere models. We examineboth spatially resolved and integrated emission-line spectra of the HII regionsDEM L323, DEM L243, DEM L199, and DEM L301 in the Large Magellanic Cloud. Thepublished spectral classifications of the dominant stars range from O7 to WN3,and morphologies range from Stromgren sphere to shell structure. Two of theobjects include SNR contamination. The overall agreement with the predictionsis generally within 0.2 dex for major diagnostic line ratios. An apparentpattern in the remaining discrepancies is that the predicted T_e is ~1000 Khotter than observed. (Abridged) Our analysis of the complex DEM L199 allows a nebular emission-line test ofunprecedented detail for WR atmospheres. Surprisingly, we find no nebular He II4686 emission, despite the fact that both of the dominant WN3 stars should behot enough to fully ionize He I in their atmospheres. We confirm that the \eta-prime emission-line parameter is not as useful ashoped for determining the ionizing stellar effective temperature, T*. Bothempirically and theoretically, we find that it is insensitive for T* >40 kK,and that it also varies spatially. The shock-contaminated objects show that\eta-prime will also yield a spuriously high T* in the presence of shocks. Itis furthermore sensitive to shell morphology. We suggest [Ne III]/Hb as anadditional probe of T*. Although it is abundance-dependent, [Ne III]/Hb hashigher sensitivity to T*, is independent of morphology, and is insensitive toshocks in our objects. We attempt a first empirical calibration of thesenebular diagnostics of T*.