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HII regions play a crucial role in the measurement of the chemicalcomposition of the interstellar medium and provide fundamental data aboutelement abundances that constrain models of galactic chemical evolution.Discrepancies that still exist between observed emission line strengths andthose predicted by nebular models can be partly attributed to the spectralenergy distributions (SEDs) of the sources of ionizing radiation used in themodels as well as simplifying assumptions made in nebular modeling. Theinfluence of stellar metallicity on nebular line strength ratios, via itseffect on the SEDs, is of similar importance as variations in the nebularmetallicity. We have computed a grid of model atmosphere SEDs for massive andvery massive O-type stars covering a range of metallicities from significantlysubsolar (0.1 Zsun) to supersolar (2 Zsun). The SEDs have been computed using astate-of-the-art model atmosphere code that takes into account the attenuationof the ionizing flux by the spectral lines of all important elements and thehydrodynamics of the radiatively driven winds and their influence on the SEDs.For the assessment of the SEDs in nebular simulations we have developed a(heretofore not available) 3d radiative transfer code that includes atime-dependent treatment of the metal ionization. Using the SEDs in both 1d and3d nebular models we explore the relative influence of stellar metallicity, gasmetallicity, and inhomogeneity of the gas on the nebular ionization structureand emission line strengths. We find that stellar and gas metallicity are ofsimilar importance for establishing the line strength ratios commonly used innebular diagnostics, whereas inhomogeneity of the gas has only a subordinateinfluence on the global line emission.

astronomy and astrophysics

Three-dimensional modeling of ionized gas. II. Spectral energy  distributions of massive and very massive stars in stationary and  time-dependent modeling of the ionization of metals in HII regions