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W51
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A tour of the W51 cloud complex

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W51 cloud complex
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W51 cloud complex: Chandra Peers into a Nurturing Cloud

The giant molecular cloud known as W51 is one of the closest to Earth at a distance of about 17,000 light years. Because of its relative proximity, W51 provides astronomers with an excellent opportunity to study how stars are forming in our Milky Way galaxy.
A new composite image of W51 shows the high-energy output from this stellar nursery, where X-rays from Chandra are coloured blue. In about 20 hours of Chandra exposure time, over 600 young stars were detected as point-like X-ray sources, and diffuse X-ray emission from interstellar gas with a temperature of a million degrees or more was also observed.

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G49.2-0.7
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Title: High velocity HI is not associated with TeV supernova remnant W51C
Authors: W.W. Tian, D.A. Leahy

The recently-detected TeV gamma-ray source HESS J1923+141 coincides with Supernova Remnant (SNR) W51C and the star forming region W51B of the W51 complex. We construct HI absorption spectra to SNR W51C, HII regions G49.2-0.35 and G49.1-0.38 in W51B, and a nearby compact extragalactic source. Our study detects high-velocity (HV) HI clouds (above 83 km/s) which coincide with W51B, but finds that the clouds are behind W51B. Both W51C and G49.2-0.35 have similar highest-velocity absorption features at ~70 km/s. The HII region G49.1-0.38 is behind the SNR because its HI absorption spectrum has a feature at 83 km/s. These new results argue against previous claims that the SNR has shocked the HV HI clouds. Therefore the TeV emission from the complex should not be associated with the HV HI clouds. W51C has a distance of about 4.3 kpc, smaller than the tangent point distance of 5.5 kpc in that direction, but still in the Sagittarius spiral arm.

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RE: Molecular Cloud W51 IRS2
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Title: Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionisation Rate
Authors: Nick Indriolo, D. A. Neufeld, M. Gerin, T. R. Geballe, J. H. Black, K. M. Menten, J. R. Goicoechea

Absorption lines from the molecules OH+, H2O+, and H3+ have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionisation rate of atomic hydrogen. H3+ is used to infer the cosmic-ray ionisation rate of H2 in the molecular interior of the cloud, which we find to be zeta_2=(4.8+-3.4)x10^-16 per second. Combining the results from all three species we find an efficiency factor---defined as the ratio of the formation rate of OH+ to the cosmic-ray ionisation rate of H---of epsilon=0.070.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionisation rate.

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Title: Trigonometric Parallax of W51 Main/South
Authors: M. Sato, M. J. Reid, A. Brunthaler, K. M. Menten
(Version v3)

We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array (VLBA). We measure a value of 0.185 0.010 mas, corresponding to a distance of 5.41 (+0.31/-0.28) kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic centre of Ro = 8.3 0.46 (statistical) 1.0 (systematic) kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star forming region. We find W51 is in a nearly circular orbit about the Galactic centre. The H2O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.

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Posts: 129897
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Title: Trigonometric Parallax of W51 Main/South
Authors: M. Sato, M. J. Reid, A. Brunthaler, K. M. Menten

We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array (VLBA). We measure a value of 0.185 0.010 mas, corresponding to a distance of 5.41 (+0.31/-0.28) kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic centre of Ro = 8.3 0.46 (statistical) 1.0 kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star forming region. We find W51 is in a nearly circular orbit about the Galactic centre. The H2O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.

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-- Edited by Blobrana on Wednesday 23rd of June 2010 02:28:15 AM

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A team led by John Lacy of the University of Texas at Austin recently used the instrument TEXES on Gemini North to map the star-forming region IRS2 located deep within the molecular cloud associated with the giant H II region W51. The spatial and spectral performance of TEXES on Gemini allowed the team to separate the ionised gas into several components for the first time. These include: an extended HII region on the front surface of the molecular cloud, several embedded compact HII regions, and a streamer of high velocity gas. The team interpreted the high velocity streamer to be a precessing or fan-like jet that has emerged from the molecular cloud into an OB star cluster where it is being ionised. The source of the jet is unknown. The presence of such a jet coming from within the molecular cloud is tantalizing evidence that there are as-yet-unseen young stars very highly embedded within the core of the cloud.

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W51 IRS2
Credit Gemini Observatory/AURA

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