10,920 research outputs found
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
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
Forming a Primordial Star in a Relic HII Region
There has been considerable theoretical debate over whether photoionization
and supernova feedback from the first Population III stars facilitate or
suppress the formation of the next generation of stars. We present results from
an Eulerian adaptive mesh refinement simulation demonstrating the formation of
a primordial star within a region ionized by an earlier nearby star. Despite
the higher temperatures of the ionized gas and its flow out of the dark matter
potential wells, this second star formed within 23 million years of its
neighbor's death. The enhanced electron fraction within the HII region
catalyzes rapid molecular hydrogen formation that leads to faster cooling in
the subsequent star forming halos than in the first halos. This "second
generation" primordial protostar has a much lower accretion rate because,
unlike the first protostar, it forms in a rotationally supported disk of
approx. 10-100 solar masses. This is primarily due to the much higher angular
momentum of the halo in which the second star forms. In contrast to previously
published scenarios, such configurations may allow binaries or multiple systems
of lower mass stars to form. These first high resolution calculations offer
insight into the impact of feedback upon subsequent populations of stars and
clearly demonstrate how primordial chemistry promotes the formation of
subsequent generations of stars even in the presence of the entropy injected by
the first stars into the IGM.Comment: 4 pages, 2 figures. Some revisions, including enhanced discussion of
angular momentum issues. Asrophysical Journal, accepte
Experimental Flow Models for SSME Flowfield Characterization
Full scale flow models with extensive instrumentation were designed and manufactured to provide data necessary for flow field characterization in rocket engines of the Space Shuttle Main Engine (SSME) type. These models include accurate flow path geometries from the pre-burner outlet through the throat of the main combustion chamber. The turbines are simulated with static models designed to provide the correct pressure drop and swirl for specific power levels. The correct turbopump-hot gas manifold interfaces were designed into the flow models to permit parametric/integration studies for new turbine designs. These experimental flow models provide a vehicle for understanding the fluid dynamics associated with specific engine issues and also fill the more general need for establishing a more detailed fluid dynamic base to support development and verification of advanced math models
Cosmology of the Next-to-Minimal Supersymmetric Standard Model
We discuss the domain wall problem in the Next-to-Minimal Supersymmetric
Standard Model, with particular attention to the usual solution of explicit
breaking of the discrete symmetry by non-renormalisable operators. This
``solution'' leads to a contradiction between the requirements of cosmology and
those of avoiding the destabilisation of the hierarchy.Comment: 6 pages LaTeX, needs sprocl.sty (included at end) Talk presented by
P.L. White at Valencia 9
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