29,441 research outputs found

    How the First Stars Regulated Local Star Formation I: Radiative Feedback

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    We present numerical simulations of how a 120 M⊙_\odot primordial star regulates star formation in nearby cosmological halos at z∼z \sim 20 by photoevaporation. Our models include nine-species primordial chemistry and self-consistent multifrequency conservative transfer of UV photons with all relevant radiative processes. Whether or not new stars form in halos clustered around a Population III star ultimately depends on their core densities and proximity to the star. Diffuse halos with central densities below 2 - 3 cm−3^{-3} are completely ionized and evaporated anywhere in the cluster. Evolved halos with core densities above 2000 cm−3^{-3} are impervious to both ionizing and Lyman-Werner flux at most distances from the star and collapse as quickly as they would in its absence. Star formation in halos of intermediate density can be either promoted or suppressed depending on how the I-front remnant shock compresses, deforms and enriches the core with H2_2. We find that the 120 M⊙_\odot star photodissociates H2_2 in most halos in the cluster but that catalysis by H- restores it a few hundred kyr after the death of the star, with little effect on star formation. Our models exhibit significant departures from previous one-dimensional spherically-symmetric simulations, which are prone to serious errors due to unphysical geometric focusing effects.Comment: 20 pages, 19 figures, accepted by ApJ, title and abstract change

    Determining the Surface-To-Bulk Progression in the Normal-State Electronic Structure of Sr2RuO4 by Angle-Resolved Photoemission and Density Functional Theory

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    In search of the potential realization of novel normal-state phases on the surface of Sr2RuO4 - those stemming from either topological bulk properties or the interplay between spin-orbit coupling (SO) and the broken symmetry of the surface - we revisit the electronic structure of the top-most layers by ARPES with improved data quality as well as ab-initio LDA slab calculations. We find that the current model of a single surface layer (\surd2x\surd2)R45{\deg} reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with LDA+SO slab calculations based on the LEED-determined surface crystal structure, reveal that (at a minimum) the sub-surface layer also undergoes a similar although weaker reconstruction. This points to a surface-to-bulk progression of the electronic states driven by structural instabilities, with no evidence for Dirac and Rashba-type states or surface magnetism.Comment: 4 pages, 4 figures, 1 table. Further information and PDF available at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/articles.htm

    Cotton, Clemency, and Control: United States v. Klein and the Juridical Legacy of Executive Pardon

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    When the guns of war fell silent in 1865, Americans throughout the reunited states grappled with the logistics of peace. At virtually every turn lay nebulous but critical questions of race, class, allegiance, and identity. More pragmatic legal stumbling blocks could also be found strewn across the path to Reconstruction; some of them would ensnare the healing nation for decades to come. Among their number was notorious Supreme Court decision United States v. Klein (1872). Born on July 22, 1865 out of a small debate over the wartime seizure of Vicksburg cotton stores, Klein quickly evolved into a legal behemoth. In its tangles with the separation of powers, the presidential power of pardon, and the supremacy of the executive in judicial matters, United States v. Klein would ultimately amount to the very poster child of the snowball effect at work in Reconstruction law. Widely forgotten or overlooked today, the decision of United States v. Klein nonetheless stands as one of the most crucial battles of the American Civil War era

    Binaural Cues for Distance and Direction of Nearby Sound Sources

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    To a first-order approximation, binaural localization cues are ambiguous: a number of source locations give rise to nearly the same interaural differences. For sources more than a meter from the listener, binaural localization cues are approximately equal for any source on a cone centered on the interaural axis (i.e., the well-known "cones of confusion"). The current paper analyzes simple geometric approximations of a listener's head to gain insight into localization performance for sources near the listener. In particular, if the head is treated as a rigid, perfect sphere, interaural intensity differences (IIDs) can be broken down into two main components. One component is constant along the cone of confusion (and thus co varies with the interaural time difference, or ITD). The other component is roughly constant for a sphere centered on the interaural axis and depends only on the relative pathlengths from the source to the two ears. This second factor is only large enough to be perceptible when sources are within one or two meters of the listener. These results are not dramatically different if one assumes that the ears are separated by 160 degrees along the surface of the sphere (rather than diametrically opposite one another). Thus, for sources within a meter of the listener, binaural information should allow listeners to locate sources within a volume around a circle centered on the interaural axis, on a "doughnut of confusion." The volume of the doughnut of confusion increases dramatically with angle between source and the interaural axis, degenerating to the entire median plane in the limit.Air Force Office of Scientific Research (F49620-98-1-0108
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