13,892 research outputs found

    Giant Molecular Clouds are More Concentrated to Spiral Arms than Smaller Clouds

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    From our catalog of Milky Way molecular clouds, created using a temperature thresholding algorithm on the Bell Laboratories 13CO Survey, we have extracted two subsets:(1) Giant Molecular Clouds (GMCs), clouds that are definitely larger than 10^5 solar masses, even if they are at their `near distance', and (2) clouds that are definitely smaller than 10^5 solar masses, even if they are at their `far distance'. The positions and velocities of these clouds are compared to the loci of spiral arms in (l, v) space. The velocity separation of each cloud from the nearest spiral arm is introduced as a `concentration statistic'. Almost all of the GMCs are found near spiral arms. The density of smaller clouds is enhanced near spiral arms, but some clouds (~10%) are unassociated with any spiral arm. The median velocity separation between a GMC and the nearest spiral arm is 3.4+-0.6 km/s, whereas the median separation between smaller clouds and the nearest spiral arm is 5.5+-0.2 km/s.Comment: 11 pages, 3 figure

    Large-Scale CO Maps of the Lupus Molecular Cloud Complex

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    Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3 transitions toward three members of the Lupus Molecular Cloud Complex - Lupus I, III, and IV - trace the column density and temperature of the molecular gas. Comparison with IR extinction maps from the c2d project requires most of the gas to have a temperature of 8-10 K. Estimates of the cloud mass from 13CO emission are roughly consistent with most previous estimates, while the line widths are higher, around 2 km/s. CO 4-3 emission is found throughout Lupus I, indicating widespread dense gas, and toward Lupus III and IV. Enhanced line widths at the NW end and along the edge of the B228 ridge in Lupus I, and a coherent velocity gradient across the ridge, are consistent with interaction between the molecular cloud and an expanding HI shell from the Upper-Scorpius subgroup of the Sco-Cen OB Association. Lupus III is dominated by the effects of two HAe/Be stars, and shows no sign of external influence. Slightly warmer gas around the core of Lupus IV and a low line width suggest heating by the Upper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an HI shell.Comment: 54 pages, 27 figures, 5 tables. To appear in ApJS. Preprint also available (with full-size figures) from http://www.astro.ex.ac.uk/people/nfht/publications.html Datacubes available from http://www.astro.ex.ac.uk/people/nfht/resources.htm

    Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy

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    Using the Westerbork Synthesis Radio Telescope, we have detected HI 21 cm-line absorption in the Warm Neutral Medium of the Galaxy toward the extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s with a full width at half maximum of 53+/-6 km/s, is associated with the Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3). The estimated spin temperature of the gas is 3600+/-360 K. The volume density is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273 and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the 3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600 K. In addition, we have also detected HI 21 cm-line absorption from high velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of -117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature components in the high velocity clouds with spin temperatures of greater than 1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table

    The Jabal Akhdar Dome in the Oman Mountains : evolution of a dynamic fracture system

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    Acknowledgments: This study was carried out within the framework of DGMK (German Society for Petroleum and Coal Science and Technology) research project 718 “Mineral Vein Dynamics Modelling,” which is funded by the companies ExxonMobil Production Deutschland GmbH, GDF SUEZ E&P Deutschland GmbH, RWE Dea AG and Wintershall Holding GmbH, within the basic research program of the WEG Wirtschaftsverband Erdo¨l- und Erdgasgewinnung e.V. We thank the companies for their financial support and their permission to publish these results. The German University of Technology in Oman (GU-Tech) is acknowledged for its logistic support. We gratefully acknowledge the reviewers Andrea Billi and Jean-Paul Breton, whose constructive reviews greatly improved the manuscriptPeer reviewedPreprin

    Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order

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    Particles dispersed in a liquid crystal above the nematic-isotropic phase transition are wetted by a surface-induced corona of paranematic order. Such coronas give rise to pronounced two-particle interactions. In this article, we report details on the analytical and numerical study of these interactions published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially demonstrate how for large particle separations the asymptotic form of a Yukawa potential arises. We show that the Yukawa potential is a surprisingly good description for the two-particle interactions down to distances of the order of the nematic coherence length. Based on this fact, we extend earlier studies on a temperature induced flocculation transition in electrostatically stabilized colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa potential to establish a flocculation diagram for a much larger range of the electrostatic parameters, namely the surface charge density and the Debye screening length. As a new feature, a kinetically stabilized dispersion close to the nematic-isotropic phase transition is found.Comment: Revtex v4.0, 16 pages, 12 Postscript figures. Accepted for publication in Phys. Rev.

    Multiple hydrodynamical shocks induced by Raman effect in photonic crystal fibres

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    We theoretically predict the occurrence of multiple hydrodynamical-like shock phenomena in the propagation of ultrashort intense pulses in a suitably engineered photonic crystal fiber. The shocks are due to the Raman effect, which acts as a nonlocal term favoring their generation in the focusing regime. It is shown that the problem is mapped to shock formation in the presence of a slope and a gravity-like potential. The signature of multiple shocks in XFROG signals is unveiled
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