23,914 research outputs found

    The effect of PPR control and dipping on village goat populations in southwest Nigeria

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    A study conducted to determine the effect of PPR control and dipping on village goat populations in southwest Nigeria. Includes data on human and goat populations, herd structure, entries and exits, reproductive performance, monthly birth, purchases, mortalities, sales and loans-out

    Dynamic behavior of an unsteady trubulent boundary layer

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    Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle

    Observations of the Extended Distribution of Ionized Hydrogen in the Plane of M31

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    We have used the Wisconsin H-Alpha Mapper (WHAM) to observe the spatially extended distribution of ionized hydrogen in M31 beyond the stellar disk. We obtained five sets of observations, centered near the photometric major axis of M31, that extend from the center of the galaxy to just off the edge of the southwestern HI disk. Beyond the bright stellar disk, but within the HI disk, weak H-alpha is detected with an intensity I(H-alpha) = 0.05 (+0.01 / -0.02) Rayleighs. Since M31 is inclined 77 degrees with respect to the line of sight, this implies that the ambient intergalactic ionizing flux onto each side of M31 is Phi_0 <= 1.6 x 10^4 photons cm^-2 s^-1. Just beyond the outer boundary of the HI disk we find no significant detection of H-alpha and place an upper limit I(H-alpha) <= 0.019 Rayleighs.Comment: To appear in ApJ Letters; 12 pages, 4 figure

    Planetological implications of mass loss from the early Sun

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    The element lithium is observed to be underabundant in the Sun by a factor of approx. equal to 100. To account for this depletion, Boothroyd et al. (Ap. J., in press 1991) proposed a model in which the Sun's zero-age-main-sequence mass was approx. 1.1 solar magnitude. If this is the explanation for the lithium depletion, then astronomical observations of F/G dwarfs in clusters suggest that the timescale for mass loss is approx. equal to 0.6 Gyr. Assuming this approximate timescale, the authors investigated several planetological implications of the astrophysical model

    Compact Radio Cores in Seyfert Galaxies

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    We have observed a sample of 157 Seyfert galaxies with a 275 km baseline radio interferometer to search for compact, high brightness temperature radio emission from the active nucleus. We obtain the surprising result that compact radio cores are much more common in Seyfert 2 than in Seyfert 1 galaxies, which at first seems to be inconsistent with orientation unification schemes. We propose a model, involving optical depth effects in the narrow-line region, which can reconcile our result with the standard unified scheme. (Accepted for publication in ApJ 1994 Sep 10)Comment: 21 pages and 7 figures, uuencoded tar-compressed postscript files, ATP18

    Fully-Coupled Simulation of Cosmic Reionization. I: Numerical Methods and Tests

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    We describe an extension of the Enzo code to enable fully-coupled radiation hydrodynamical simulation of inhomogeneous reionization in large (100Mpc)3\sim (100 Mpc)^3 cosmological volumes with thousands to millions of point sources. We solve all dynamical, radiative transfer, thermal, and ionization processes self-consistently on the same mesh, as opposed to a postprocessing approach which coarse-grains the radiative transfer. We do, however, employ a simple subgrid model for star formation which we calibrate to observations. Radiation transport is done in the grey flux-limited diffusion (FLD) approximation, which is solved by implicit time integration split off from the gas energy and ionization equations, which are solved separately. This results in a faster and more robust scheme for cosmological applications compared to the earlier method. The FLD equation is solved using the hypre optimally scalable geometric multigrid solver from LLNL. By treating the ionizing radiation as a grid field as opposed to rays, our method is scalable with respect to the number of ionizing sources, limited only by the parallel scaling properties of the radiation solver. We test the speed and accuracy of our approach on a number of standard verification and validation tests. We show by direct comparison with Enzo's adaptive ray tracing method Moray that the well-known inability of FLD to cast a shadow behind opaque clouds has a minor effect on the evolution of ionized volume and mass fractions in a reionization simulation validation test. We illustrate an application of our method to the problem of inhomogeneous reionization in a 80 Mpc comoving box resolved with 320033200^3 Eulerian grid cells and dark matter particles.Comment: 32 pages, 23 figures. ApJ Supp accepted. New title and substantial revisions re. v
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