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Published July 20, 2002 | Submitted + Published
Journal Article Open

X‐Ray–emitting Young Stars in the Orion Nebula


The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hr exposure in two observations. We detect 1075 X-ray sources, most with subarcsecond positional accuracy. Ninety-one percent of the sources are spatially associated with known stellar members of the cluster, and an additional 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main-sequence stellar population and covers the initial mass function from brown dwarfs up to a 45 M☉ O star. Source luminosities span 5 orders of magnitude from log L_X ≃ 28.0 to 33.3 ergs s^(-1) in the 0.5-8 keV band, plasma energies range from 0.2 to >10 keV, and absorption ranges from log N_H < 20.0 to ~23.5 cm^(-2). Comprehensive tables providing X-ray and stellar characteristics are provided electronically. We examine here the X-ray properties of Orion young stars as a function of mass; other studies of astrophysical interest will appear in companion papers. Results include: (a) the discovery of rapid variability in the O9.5 31 M☉ star θ^2A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small shocks distributed throughout the radiatively accelerated wind; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G through M type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the "saturation" level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray emission; (e) evidence that T Tauri plasma temperatures are often very high with T ≥ 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main-sequence very low-mass objects showing flaring levels similar to those seen in more massive T Tauri stars and a decline in magnetic activity as they evolve into L and T type brown dwarfs.

Additional Information

© 2002 The American Astronomical Society. Received 2001 September 12; accepted 2002 March 29. We thank George Chartas and Konstantin Getman (Penn State) for expert assistance with data analysis; William Herbst (Wesleyan) for access to rotation data prior to publication; participants of the 12th Cool Star Workshop (including Gibor Basri of Berkeley, James Liebert of Arizona, and Hans Zinnecker of Potsdam) for thoughtful comments; advice from Deepto Chakrabarty (MIT), David Cohen (Swarthmore), and George Pavlov (Penn State) concerning unusual sources; and Thierry Montmerle (Saclay) and the anonymous referee for helpful critiques of the entire manuscript. This work was principally supported by NASA grant NAS 8-38252 (Garmire, PI). Additional support was obtained from the Japan Society for the Promotion of Science (Y. T.) and the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (S. H. P.). The study benefited from on-line databases including SIMBAD, Astronomical Data Center and the Astrophysics Data System.

Attached Files

Published - Feigelson_2002_ApJ_574_258.pdf

Submitted - 0203316.pdf


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