Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-10-20
Astrophys.J.655:299-315,2007
Astronomy and Astrophysics
Astrophysics
42 pages, 10 figures. To be published in ApJ
Scientific paper
10.1086/510151
We are engaged in a comprehensive program to find reliable elemental abundances in and to 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 ultraviolet FOS and STIS observations, 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 in accord with caseB 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.
Baldwin John A.
Blagrave K. P. M.
Dufour Reginald James
Hester Jeff J.
Martin Peter G.
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