2007ApJ...655..299B

We are engaged in a comprehensive program to find reliable elemental abundances in and probe the physical structure of the Orion Nebula, the brightest and best-resolved H II region. In the course of developing a robust extinction correction covering our optical and UV FOS and STIS observations, we examined the decrement within various series of He I lines. The decrements of the 2³S-n³P, 2³P-n³S, and 3³S-n³P series are not in accord with case B recombination theory. None of these anomalous He I decrements can be explained by extinction, indicating the presence of additional radiative transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY photoionization equilibrium models including radiative transfer are developed to predict the observed He I decrements, and the quantitative agreement is quite remarkable. Following from these results, select He I lines are combined with H I and [O II] lines and stellar extinction data to validate a new normalizable analytic expression for the wavelength dependence of the extinction. In so doing, the He⁺/H⁺ abundance is also derived.