We present new observations of the deuterium and hydrogen Balmer lines in the Orion Nebula. There is a real variation in the deuterium-to-hydrogen line ratios across the nebula, being greatest in the emission from the largest proplyd (Orion 244-440). We also present the results of a detailed model for the emission of these lines, the hydrogen lines being the result of photoionization and recombination while the deuterium lines are produced by fluorescent excitation of the upper energy states by the far-UV radiation from θ1 Ori C. Comparison of the observations and predictions of the line intensities shows good agreement, both in the strength of the reference lines at Hβ and also in the differences of the Balmer decrement for the two atoms. The fact that both the deuterium and hydrogen emissions arise from mechanisms that count the near-ultraviolet (deuterium) and photoionizing ultraviolet (hydrogen) photons from the dominant star means that there is little prospect of similar observations being useful for determination of D/H abundances in H II regions. Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on observations obtained at the Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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Published in The Astrophysical Journal, v. 556, no. 1, p. 203-214.

© 2001. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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