21,267 research outputs found
THINKING OUTSIDE THE BOX: BUILDING MATERIALS AND OTHER PRODUCTS FROM ANIMAL PROCESSED FIBER
Agribusiness, Livestock Production/Industries,
On open 3-manifolds proper homotopy equivalent to geometrically-simply connected polyhedra
We prove that an open 3-manifold proper homotopy equivalent to a
geometrically simply connected polyhedron is simply connected at infinity,
generalizing a theorem of V.Poenaru.Comment: revised version, 11 pages, Topology Appl. (to appear
Achieving Extreme Resolution in Numerical Cosmology Using Adaptive Mesh Refinement: Resolving Primordial Star Formation
As an entry for the 2001 Gordon Bell Award in the "special" category, we
describe our 3-d, hybrid, adaptive mesh refinement (AMR) code, Enzo, designed
for high-resolution, multiphysics, cosmological structure formation
simulations. Our parallel implementation places no limit on the depth or
complexity of the adaptive grid hierarchy, allowing us to achieve unprecedented
spatial and temporal dynamic range. We report on a simulation of primordial
star formation which develops over 8000 subgrids at 34 levels of refinement to
achieve a local refinement of a factor of 10^12 in space and time. This allows
us to resolve the properties of the first stars which form in the universe
assuming standard physics and a standard cosmological model. Achieving extreme
resolution requires the use of 128-bit extended precision arithmetic (EPA) to
accurately specify the subgrid positions. We describe our EPA AMR
implementation on the IBM SP2 Blue Horizon system at the San Diego
Supercomputer Center.Comment: 23 pages, 5 figures. Peer reviewed technical paper accepted to the
proceedings of Supercomputing 2001. This entry was a Gordon Bell Prize
finalist. For more information visit http://www.TomAbel.com/GB
Josef Meixner: his life and his orthogonal polynomials
This paper starts with a biographical sketch of the life of Josef Meixner.
Then his motivations to work on orthogonal polynomials and special functions
are reviewed. Meixner's 1934 paper introducing the Meixner and
Meixner-Pollaczek polynomials is discussed in detail. Truksa's forgotten 1931
paper, which already contains the Meixner polynomials, is mentioned. The paper
ends with a survey of the reception of Meixner's 1934 paper.Comment: v4: 18 pages, generating function for Krawtchouk polynomials on p.10
correcte
The Formation of the First Star in the Universe
We describe results from a fully self-consistent three dimensional
hydrodynamical simulation of the formation of one of the first stars in the
Universe. Dark matter dominated pre-galactic objects form because of
gravitational instability from small initidal density perturbations. As they
assemble via hierarchical merging, primordial gas cools through ro-vibrational
lines of hydrogen molecules and sinks to the center of the dark matter
potential well. The high redshift analog of a molecular cloud is formed. When
the dense, central parts of the cold gas cloud become self-gravitating, a dense
core of approximately 100 solar mass undergoes rapid contraction. At densities
n>10^9 cm^-3 a one solar mass proto-stellar core becomes fully molecular due to
three-body H_2 formation. Contrary to analytical expectations this process does
not lead to renewed fragmentation and only one star is formed. The calculation
is stopped when optical depth effects become important, leaving the final mass
of the fully formed star somewhat uncertain. At this stage the protostar is
acreting material very rapidly (~0.01 solar masses per year). Radiative
feedback from the star will not only halt its growth but also inhibit the
formation of other stars in the same pre-galactic object (at least until the
first star ends its life, presumably as a supernova). We conclude that at most
one massive (M >> 1 solar mass) metal free star forms per pre-galactic halo,
consistent with recent abundance measurements of metal poor galactic halo
stars.Comment: 8 pages, 5 figures. Science. Published in Science Express online on
11/15/2001. More information can be found at http://www.TomAbel.com/GB
- …