2006/10/20 by K. Blagrave, K. P. M. Blagrave, P. G. Martin +6 · 1 citation
Chemistry · Earth and Planetary Sciences · Physics and Astronomy · #Astrophysics #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Biology #Chemistry #Extinction (optical mineralogy) #Ion #Ionization #Lambda #Molar absorptivity #Nebula #Optics #Orion Nebula #Photoionization #Physics #Radiative equilibrium #Radiative transfer #Recombination #Series (stratigraphy) #Spectroscopy and Laser Applications #Stars #Wavelength #astro-ph
paper · pdf · doi:10.1086/510151
published as Astrophys.J.655:299-315,2007 · 42 pages, 10 figures. To be published in ApJ
arxiv created 2006/10/20 · openalex publication_date 2007/01/20 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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 2 3 S - n 3 P , 2 3 P - n 3 S , and 3 3 S - n 3 P 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.