Iron Fluorescent Line Emission from Young Stellar Objects in the Orion Nebula

Masahiro Tsujimoto, The Pennsylvania State University
Eric D. Feigelson, The Pennsylvania State University
Nicolas Grosso, Universite Joseph-Fourier
G. Micela, INAF, Osservatorio Astronomico di Palermo
Y. Tsuboi, Chuo University
F. Favata, ESTEC
H. Shang, Academia Sinica
Joel H. Kastner, Rochester Institute of Technology

This is the pre-print of an article published by the American Astronomical Society. The final, published version is available here:

© 2005 The American Astronomical Society

Also archived in: arXiv: astro-ph/0412608 v2 25 Dec 2004

The authors express gratitude for Konstantin Getman for his help in data reduction, and Salvatore Sciortino and Koji Mori for improving the manuscript. COUP is supported by Chandra guest observer grant SAO GO3-4009A (E. D. Feigelson, PI) and the ACIS Team contract NAS8-38252 (G. P. Garmire, PI). M.T. acknowledges financial supported by the Japan Society for the Promotion of Science.

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We present the result of a systematic search for the iron K-alpha fluorescent line at ~6.4keV among 1616 X-ray sources detected by ultra-deep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at ~6.4keV among 127 control sample sources with significant counts in the 6.0–9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intense flare-like flux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the fluorescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of & 1×10^22 cm^−2 and NIR excess emission, which is not expected when the fluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares (Refer to PDF file for exact formulas).