Deviations from HeiCase B Recombination Theory and Extinction Corrections in the Orion Nebula

We are engaged in a comprehensive program to find reliable elementalabundances in and to probe the physical structure of the Orion Nebula, thebrightest and best-resolved H II region. In the course of developing a robustextinction correction covering our optical and ultraviolet FOS and STISobservations, we examined the decrement within various series of He I lines.The decrements of the 2^3S-n^3P, 2^3P-n^3S and 3^3S-n^3P series are not inaccord with caseB recombination theory. None of these anomalous He I decrementscan be explained by extinction, indicating the presence of additional radiativetransfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDYphotoionization equilibrium models including radiative transfer are developedto predict the observed He I decrements and the quantitative agreement is quiteremarkable. Following from these results, select He I lines are combined with HI and [O II] lines and stellar extinction data to validate a new normalizableanalytic expression for the wavelength dependence of the extinction. In sodoing, the He+/H+ abundance is also derived.Comment: 42 pages, 10 figures. To be published in Ap