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TOPIC: Orion Nebula


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The Orion Bar
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Title: Rotationally Warm Molecular Hydrogen in the Orion Bar
Authors: Gargi Shaw, G. J. Ferland, W. J. Henney, P. C. Stancil, N. P. Abel, E.W. Pellegrini, J.A. Baldwin, P. A. M. van Hoof

The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400 to 700 K). However, standard models of PDRs are unable to reproduce such warm rotational temperatures. In this paper we attempt to explain these observations with new comprehensive models which extend from the H+ region through the Bar and include the magnetic field in the equation of state. We adopt the model parameters from our previous paper which successfully reproduced a wide variety of spectral observations across the Bar. In this model the local cosmic-ray density is enhanced above the galactic background, as is the magnetic field, and which increases the cosmic-ray heating elevating the temperature in the molecular region. The pressure is further enhanced above the gas pressure in the H+ region by the momentum transferred from the absorbed starlight. Here we investigate whether the observed H2 lines can be reproduced with standard assumptions concerning the grain photoelectric emission. We also explore the effects due to the inclusion of recently computed H2 + H2, H2 + H and H2 + He collisional rate coefficients.

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RE: Orion Nebula
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Dozens of high-speed jets of gas in a crowded stellar nursery have been traced to the stars from which they emanate.
The well-known Orion nebula, which sits some 1300 light years from Earth, is just a small portion of a much larger stellar nursery. This entire "molecular cloud" spans a region on the sky as wide as 20 full moons but is largely obscured by gas and dust.

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Astronomers using the United Kingdom Infrared Telescope (UKIRT) in Hawaii, the IRAM Millimetre-wave Telescope in Spain, and the Spitzer Space Telescope in orbit above the Earth, have completed the most wide-ranging census ever produced of dynamical star formation in and around the well-known Great Nebula of Orion. They have found this stellar nursery to be a lively and somewhat overcrowded place, with young stars emitting gas jets in all directions, creating quite a chaotic picture. There is much more going on in Orion than previously thought.
The research team comprises more than a dozen astronomers from the US, the UK and a number of other European countries. The project thus has a truly international flavour, representing a collaboration of minds from across the globe. A number of them are in Hertfordshire in the UK this week to share their discoveries with colleagues at this year's annual National Astronomy Meeting of the UK (NAM 2009).

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Theta 1 Orionis C
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A team of astronomers, led by Stefan Kraus and Gerd Weigelt from the Max-Planck-Institute for Radio Astronomy (MPIfR) in Bonn, used ESO's Very Large telescope Interferometer (VLTI) to obtain the sharpest ever image of the young double star Theta 1 Ori C in the Orion Trapezium Cluster, the most massive star in the nearest high-mass star-forming region. The new image clearly separates the two young, massive stars of this system. The observations have a spatial resolution of about 2 milli-arcseconds, corresponding to the apparent size of a car on the surface of the moon. The team was able to derive the properties of the orbit of this binary system, including the masses of the two stars (38 and 9 solar masses) and their distance from us (1350 light-years). The results show the fascinating new possibilities of high-resolution stellar imaging achievable with infrared interferometry.

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Astronomers have captured the sharpest image of a young binary star in the heart of Orion, in which one can clearly distinguish the two stars of the system.
The new image of the double star, Theta 1 Orionis C, was taken with the European Southern Observatory's Very Large Telescope Interferometer (VLTI), astronomers from the Max-Planck-Institute for Radio Astronomy in Bonn, Germany, reported in the journal Astronomy and Astrophysics on Thursday.

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Title: Tracing the young massive high-eccentricity binary system Theta1 Orionis C through periastron passage
Authors: S. Kraus, G. Weigelt, Y. Y. Balega, J. A. Docobo, K.-H. Hofmann, T. Preibisch, D. Schertl, V. S. Tamazian, T. Driebe, K. Ohnaka, R. Petrov, M. Schoeller, and M. Smith

Astronomy&Astrophysics, 2009, vol. 497, p. 195 (PDF)

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Theta1 Orionis C
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Credit ESO

VLTI image showing the double star system Theta1 Orionis C in the Orion Nebula Trapezium.
From these observations, a team of astronomers, led by Stefan Kraus and Gerd Weigelt from the Max-Planck Institute in Bonn, could obtain the full orbit of the two stars in the system, and derive the total mass of the two stars (47 solar masses) and their distance from us (1350 light-years).

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RE: Orion Nebula
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Title: The three dimensional dynamic structure of the inner Orion Nebula
Authors: C. R. O'Dell (Vanderbilt), W. J. Henney (CRyA, UNAM, Mexico), N. P. Abel (Cincinnati), G. J. Ferland (Kentucky), S. J. Arthur (CRyA, UNAM, Mexico)

The three dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and new data. We find that the widely accepted model of a concave blister of ionised material needs to be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate cloud of very optically thick molecules within the body of ionised gas, which is probably the location of the multiple embedded sources that produce the outflows that define the Orion-S star formation region. Evidence for this cloud comes from the presence of H2CO lines in absorption in the radio continuum and discrepancies in the extinction derived from radio-optical and optical only emission. We present an equilibrium Cloudy model of the Orion-S cloud, which successfully reproduces many observed properties of this feature. We also report the discovery of an open-sided shell of [O III] surrounding the Trapezium stars, revealed through emission line ratio images and the onset of radiation shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind from theta^1 Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of theta^1 Ori C with respect to the Orion Molecular Cloud, which could mean that this star has only recently begun to photoionise the Orion Nebula. We show that current observations of the Nebula do not rule out such a possibility, so long as the ionisation front has propagated into a pre-existing low-density region. In addition, a young age for the Nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.

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Million-Degree Plasma Pervading the Extended Orion Nebula
Four extremely bright and massive stars, called the Trapezium, primarily illuminate the Orion Nebula.
This heating is likely the result of shocks from powerful outflows from one bright star in the Trapezium. The majority of the stars in our Galaxy are found in regions similar to the Orion Nebula, so this phenomenon should be widespread throughout the galactic plane.

trapOri_e1
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This mosaic image depicts a 12.8 square light-year portion of the Orion Nebula, the nearest region of massive-star formation. The bright area in the upper left centre is the well-studied Trapezium region of 9000 K gas, and the much larger nearly circular region to the lower right is the Extended Orion Nebula within which diffuse x-ray emission from 2 million K gas was observed by Güdel et al.

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