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RE: Orion Nebula Cluster
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Image of the Orion Trapezium Cluster captured with a 8" f5 reflector and Canon EOS 350D. 

Picture 222 


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Title: The Hubble Space Telescope Treasury Program on the Orion Nebula Cluster
Authors: M. Robberto (STScI), D. R. Soderblom (STScI), E. Bergeron (STScI), V. Kozhurina-Platais (STScI), R. B. Makidon (STScI), P. R. McCullough (STScI), M. McMaster (STScI), N. Panagia (STScI), I. N. Reid (STScI), Z. Levay (STScI), L. Frattare (STScI), N. Da Rio (ESA-ESTEC), M. Andersen (ESA-ESTEC), C. R. O'Dell (Vanderbilt), K. G. Stassun (Vanderbilt, Fisk University, MIT), M. Simon (Stony Brook), E. D. Feigelson (Pennsylvania State), J. R. Stauffer (Spitzer Science Centre), M. Meyer (ETH-Zurich), M. Reggiani (ETH-Zurich), J. Krist (JPL), C. F. Manara (ESO), M. Romaniello (ESO), L. A. Hillenbrand (Caltech), L. Ricci (Caltech), F. Palla (INAF-Arcetri), J. R. Najita (NOAO), T. T. Ananna (Bryn Mawr College), G. Scandariato (INAF-Catania), K. Smith (MPIA-Heidelberg)

The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster has used 104 orbits of HST time to image the Great Orion Nebula region with the Advanced Camera for Surveys (ACS), the Wide-Field/Planetary Camera 2 (WFPC2) and the Near Infrared Camera and Multi Object Spectrograph (NICMOS) instruments in 11 filters ranging from the U-band to the H-band equivalent of HST. The program has been intended to perform the definitive study of the stellar component of the ONC at visible wavelengths, addressing key questions like the cluster IMF, age spread, mass accretion, binarity and cirumstellar disk evolution. The scanning pattern allowed to cover a contiguous field of approximately 600 square arcminutes with both ACS and WFPC2, with a typical exposure time of approximately 11 minutes per ACS filter, corresponding to a point source depth AB(F435W) = 25.8 and AB(F775W)=25.2 with 0.2 magnitudes of photometric error. We describe the observations, data reduction and data products, including images, source catalogues and tools for quick look preview. In particular, we provide ACS photometry for 3399 stars, most of them detected at multiple epochs, WFPC2 photometry for 1643 stars, 1021 of them detected in the U-band, and NICMOS JH photometry for 2116 stars. We summarize the early science results that have been presented in a number of papers. The final set of images and the photometric catalogues are publicly available through the archive as High Level Science Products at the STScI Multimission Archive hosted by the Space Telescope Science Institute.

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YSO CHS7797
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Title: The spectral type of CHS7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster
Authors: M.V. Rodriguez-Ledesma (1 and 2), R. Mundt (1), O. Pintado (3), S. Boudreault (4), F. Hessman (2), W. Herbst (5) ((1) MPIA, (2) Goettingen Univ., (3) INSUGEO, (4) IAC, (5) Wesleyan)

We present the spectroscopic characterisation of the unusual high-amplitude very low mass pre-main-sequence periodic variable CHS7797. This study is based on optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range, carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have been carried out at two distinct phases of the 17.8d period, namely at maximum and four days before maximum. Four different spectral indices were used for the spectral classification at these two phases, all of them well-suited for spectral classification of young and obscured late M dwarfs. In addition, the gravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines were used to confirm the young age of CHS7797. From the spectrum obtained at maximum light we derived a spectral type (SpT) of M6.05, while for the spectrum taken four days before maximum the derived SpT is M5.75. The derived SpTs confirm that CHS7797 has a mass in the stellar-substellar boundary mass range. In addition, the small differences in the derived SpTs at the two observed phases may provide indirect hints that CHS7797 is a binary system of similar mass components surrounded by a tilted circumbinary disk, a system similar to KH15D.

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Astrophysicists locate black hole close to Earth

Astrophysicists have cracked the mystery of the binding force behind a cluster of unruly and rapidly swirling stars located in the famous Sword of Orion. Using sophisticated computer modelling, they found these fast-moving stars, known as the Orion Nebula Cluster, were potentially held together by the gravitational pull of a black hole up to 200 times the mass of the Sun.
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Trapezium Cluster
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Title: High Resolution Images of Orbital Motion in the Orion Trapezium Cluster with the LBT Adaptive Optics System
Authors: L.M. Close, A. Puglisi, J.R. Males, C. Arcidiacono, A. Skemer, J.C. Guerra, L. Busoni, G. Brusa, E. Pinna, D.L. Miller, A. Riccardi, D.W. McCarthy, M. Xompero, C. Kulesa, F. Quiros-Pacheco, J. Argomedo, J. Brynnel, S. Esposito, F. Mannucci, K. Boutsia, L. Fini, D.J. Thompson, J.M. Hill, C.E. Woodward, R. Briguglio, T.J. Rodigas, P. Stefanini, G. Agapito, P. Hinz, K. Follette, R. Green

The new 8.4m LBT adaptive secondary AO system, with its novel pyramid wavefront sensor, was used to produce very high Strehl (75% at 2.16 microns) near infrared narrowband (Br gamma: 2.16 microns and [FeII]: 1.64 microns) images of 47 young (~1 Myr) Orion Trapezium theta1 Ori cluster members. The inner ~41x53" of the cluster was imaged at spatial resolutions of ~0.050" (at 1.64 microns). A combination of high spatial resolution and high S/N yielded relative binary positions to ~0.5 mas accuracies. Including previous speckle data, we analyse a 15 year baseline of high-resolution observations of this cluster. We are now sensitive to relative proper motions of just ~0.3 mas/yr (0.6 km/s at 450 pc) this is a ~7x improvement in orbital velocity accuracy compared to previous efforts. We now detect clear orbital motions in the theta1 Ori B2/B3 system of 4.9±0.3 km/s and 7.2±0.8 km/s in the theta1 Ori A1/A2 system (with correlations of PA vs. time at >99% confidence). All five members of the theta1 Ori B system appear likely as a gravitationally bound "mini-cluster". The very lowest mass member of the theta1 Ori B system (B4; mass ~0.2 Msun) has, for the first time, a clearly detected motion (at 4.3±2.0 km/s; correlation=99.7%) w.r.t B1. However, B4 is most likely in an long-term unstable (non-hierarchical) orbit and may "soon" be ejected from this "mini-cluster". This "ejection" process could play a major role in the formation of low mass stars and brown dwarfs.

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Title: Gravitational Slingshot of Young Massive Stars in Orion
Authors: Sourav Chatterjee, Jonathan C. Tan

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest amongst the ONC's ~1000 members are: \theta^1 Ori C, the most massive binary in the cluster with stars of masses 38 and 9 solar masses (Kraus et al. 2009); the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of ~8 solar masses (Tan 2004); and the Kleinmann-Low (KL) nebula protostar, a highly-obscured, ~15 solar mass object still accreting gas while also driving a powerful, apparently 'explosive' outflow (Allen & Burton 1993). The unusual behaviour of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here we report the results of a systematic survey using ~10^7 numerical experiments of gravitational interactions of the \theta1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for \theta1C. Several other observed properties of \theta1C are also consistent with it having ejected BN and altogether we estimate there is only a ~10^{-5} probability that \theta1C has these properties by chance. Our results suggest that after being launched from \theta1C 4,500 years ago, BN has ploughed through the KL massive-star-forming core within the last 1,000 years causing its recently-enhanced accretion and outflow activity.

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Orion BN/KL region
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Title: First Science Observations with SOFIA/FORCAST: 6 TO 37 micron Imaging of Orion BN/KL
Authors: James M. De Buizer (1), Mark R. Morris (2), E. E. Becklin (1,2), Hans Zinnecker (1), Terry L. Herter (3), Joseph D. Adams (3), Ralph Y. Shuping (4,1), William D. Vacca (1) ((1) SOFIA-USRA, (2) UCLA, (3) Cornell, (4) Space Science Institute)

The BN/KL region of the Orion Nebula is the nearest region of high mass star formation in our galaxy. As such, it has been the subject of intense investigation at a variety of wavelengths, which have revealed it to be brightest in the infrared to sub-mm wavelength regime. Using the newly commissioned SOFIA airborne telescope and its 5-40 micron camera FORCAST, images of the entire BN/KL complex have been acquired. The 31.5 and 37.1 micron images represent the highest resolution observations (31.5 microns, and that this distinction goes instead to the source IRc4. It was determined from these images and derived dust colour temperature maps that IRc4 is also likely to be self-luminous. A new source of emission has also been identified at wavelengths >31.5 microns that coincides with the northeastern outflow lobe from the protostellar disk associated with radio source I.

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Orion Nebula Cluster
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Title: A Wide-Field Survey of the Orion Nebula Cluster in the Near-Infrared
Authors: M. Robberto, D. R. Soderblom, G. Scandariato, K. Smith, N. Da Rio, I. Pagano, L. Spezzi

We present J, H and K photometry of the Orion Nebula Cluster obtained at the CTIO/Blanco 4 m telescope in Cerro Tololo with the ISPI imager. From the observations we have assembled a catalogue of about 7800 sources distributed over an area of approximately 30'x40', the largest of any survey deeper than 2MASS in this region. The catalogue provides absolute coordinates accurate to about 0.15 arcseconds and 3sigma photometry in the 2MASS system down to J 19.5mag, H 18.0mag, K 18.5mag, enough to detect planetary size objects 1 Myr old under Av 10mag of extinction at the distance of the Orion Nebula. We present a preliminary analysis of the catalog, done comparing the (J-H, H-K) colour-colour diagram, the (H, J-H) and (K, H-K) colour-magnitude diagrams and the JHK luminosity functions of three regions at increasing projected distance from the Trapezium. Sources in the inner region typically show IR colours compatible with reddened T Tauri stars, whereas the outer fields are dominated by field stars seen through an amount of extinction which decreases with the distance from the center. The colour-magnitude diagrams make it possible to clearly distinguish between the main ONC population, spread across the full field, and background sources. The luminosity functions of the inner region, corrected for completeness, remain relatively flat in the sub-stellar regime regardless of the strategy adopted to remove background contamination.

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A detailed survey of stars in the Orion Nebula has found that fewer than 10 percent have enough surrounding dust to make Jupiter-sized planets, according to a report by astronomers at the University of California, Berkeley, the California Institute of Technology (Caltech) and the Harvard-Smithsonian Centre for Astrophysics.
Because stars like the sun probably formed in hot open clusters like Orion, the finding suggests that sun-like stars have a low probability of forming planets, or at least, planets the size of Jupiter or larger.

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Title: Visual Binaries in the Orion Nebula Cluster
Authors: Bo Reipurth, Marcelo M. Guimarăes, Michael S. Connelley, John Bally

We have carried out a major survey for visual binaries towards the Orion Nebula Cluster using HST images obtained with an H-alpha filter. Among 781 likely ONC members more than 60" from theta-1 Ori C, we find 78 multiple systems (75 binaries and 3 triples), of which 55 are new discoveries, in the range from 0.1" to 1.5". About 9 binaries are likely line-of-sight associations. We find a binary fraction of 8.8%+-1.1% within the limited separation range from 67.5 to 675 AU. The field binary fraction in the same range is a factor 1.5 higher. Within the range 150 AU to 675 AU we find that T Tauri associations have a factor 2.2 more binaries than the ONC. The binary separation distribution function of the ONC shows unusual structure, with a sudden steep decrease in the number of binaries as the separation increases beyond 0.5", corresponding to 225 AU. We have measured the ratio of binaries wider than 0.5" to binaries closer than 0.5" as a function of distance from the Trapezium, and find that this ratio is significantly depressed in the inner region of the ONC. The deficit of wide binaries in the central part of the cluster is likely due to dissolution or orbital change during their passage through the potential well of the inner cluster region. Many of the companions are likely to be brown dwarfs.

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