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

Wepresenttheresultofasystematic searchfortheironKfluorescentline at � 6.4keVamong1616X-ray sources detected by ultradeep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultradeep Project (COUP). Seven sources are identified to have an excess emission at � 6.4 keV 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 intenseflarelikeflux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the florescence 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 k1 ; 10 22 cm � 2 and NIR excess emission, which is not expected when thefluorescence 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. Subject headingg ISM:individual(OMC-1) — openclustersandassociations:individual(OrionNebulaCluster) — scattering — stars: pre-main-sequence — X-rays: stars

[1]  J. Scargle.  Bayesian Blocks, A New Method to Analyze Structure in Photon Counting Data , 1997, astro-ph/9711233.

[2]  Infrared L-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars , 2000, astro-ph/0008280.

[3]  Andrew C. Fabian,et al.  X-ray reflection from cold matter in Active Galactic Nuclei and X-ray binaries , 1991 .

[4]  M. C. Weisskopf,et al.  An Overview of the Performance and Scientific Results from the Chandra X‐Ray Observatory , 2001, astro-ph/0110308.

[5]  L. House.  Theoretical Wavelengths for Ka-TYPE X-Ray Lines in the Spectra of Ionized Atoms (carbon to Copper) , 1969 .

[6]  M. Meyer,et al.  Intrinsic near-infrared excesses of T tauri stars: Understanding the classical T tauri star locus , 1997 .

[7]  Kazunori Ishibashi,et al.  The ASCA X-Ray Spectrum of η Carinae , 1998 .

[8]  Mark W. Bautz,et al.  Characteristics of Diffuse X-Ray Line Emission within 20 Parsecs of the Galactic Center , 2003, astro-ph/0311460.

[9]  E. Feigelson,et al.  X-Ray-emitting Young Stars in the Orion Nebula , 2002, astro-ph/0203316.

[10]  M. Tsujimoto,et al.  Chandra Orion Ultradeep Project: Observations and Source Lists , 2004, astro-ph/0410136.

[11]  B. Krauskopf,et al.  Proc of SPIE , 2003 .

[12]  Duane A. Liedahl,et al.  New Calculations of Fe L-Shell X-Ray Spectra in High-Temperature Plasmas , 1995 .

[13]  R. D. Cowan,et al.  Iron-Line Emission During Solar Flares. , 1971 .

[14]  John K. Tomfohr,et al.  Lecture Notes on Physics , 1879, Nature.

[15]  W. J. Gates,et al.  Observation of the solar flare X-ray emission line spectrum of iron from 1.3 to 20 angstrom units. , 1967 .

[16]  C. J. Wolfson,et al.  SMM observations of K-alpha radiation from fluorescence of photospheric iron by solar flare X-rays , 1984 .

[17]  T. Bai.  Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance , 1979 .

[18]  J. Krolik,et al.  Fe K Features as Probes of the Nuclear Reflection Region in Seyfert Galaxies , 1987 .

[19]  The Luminosity and Mass Function of the Trapezium Cluster: From B Stars to the Deuterium-burning Limit , 2002, astro-ph/0203122.

[20]  John E. Davis.  Event Pileup in Charge-coupled Devices , 2001 .

[21]  K. Makishima.  Iron lines from galactic and extragalactic X-ray sources , 1986 .

[22]  Yasuo Tanaka,et al.  ASCA View of Our Galactic Center: Remains of Past Activities in X-Rays? , 1996 .

[23]  Mark W. Bautz,et al.  Advanced CCD imaging spectrometer (ACIS) instrument on the Chandra X-ray Observatory , 2003, SPIE Astronomical Telescopes + Instrumentation.

[24]  S. Borgani,et al.  First Results from the X-Ray and Optical Survey of the Chandra Deep Field South , 2000, astro-ph/0007240.

[25]  J. Stutzki,et al.  The Orion Molecular Cloud and Star-Forming Region , 1988 .

[26]  Sergei Nayakshin,et al.  Accretion Disk Models and Their X-Ray Reflection Signatures. I. Local Spectra , 2000, astro-ph/0005597.

[27]  A. Cox,et al.  Allen's astrophysical quantities , 2000 .

[28]  K. Koyama,et al.  Chandra Observation of the ρ Ophiuchi Cloud , 2001 .

[29]  Jeffrey D. Scargle.  Studies in Astronomical Time Series Analysis: V. Bayesian Blocks, A New Method to Analyze Structure in , 1998 .