1,851 research outputs found
Cauchy-perturbative matching revisited: tests in spherical symmetry
During the last few years progress has been made on several fronts making it
possible to revisit Cauchy-perturbative matching (CPM) in numerical relativity
in a more robust and accurate way. This paper is the first in a series where we
plan to analyze CPM in the light of these new results.
Here we start by testing high-order summation-by-parts operators, penalty
boundaries and contraint-preserving boundary conditions applied to CPM in a
setting that is simple enough to study all the ingredients in great detail:
Einstein's equations in spherical symmetry, describing a black hole coupled to
a massless scalar field. We show that with the techniques described above, the
errors introduced by Cauchy-perturbative matching are very small, and that very
long term and accurate CPM evolutions can be achieved. Our tests include the
accretion and ring-down phase of a Schwarzschild black hole with CPM, where we
find that the discrete evolution introduces, with a low spatial resolution of
\Delta r = M/10, an error of 0.3% after an evolution time of 1,000,000 M. For a
black hole of solar mass, this corresponds to approximately 5 s, and is
therefore at the lower end of timescales discussed e.g. in the collapsar model
of gamma-ray burst engines.
(abridged)Comment: 14 pages, 20 figure
Bundles of Interacting Strings in Two Dimensions
Bundles of strings which interact via short-ranged pair potentials are
studied in two dimensions. The corresponding transfer matrix problem is solved
analytically for arbitrary string number N by Bethe ansatz methods. Bundles
consisting of N identical strings exhibit a unique unbinding transition. If the
string bundle interacts with a hard wall, the bundle may unbind from the wall
via a unique transition or a sequence of N successive transitions. In all
cases, the critical exponents are independent of N and the density profile of
the strings exhibits a scaling form that approaches a mean-field profile in the
limit of large N.Comment: 8 pages (latex) with two figure
Electrophoretic mobility of a charged colloidal particle: A computer simulation study
We study the mobility of a charged colloidal particle in a constant
homogeneous electric field by means of computer simulations. The simulation
method combines a lattice Boltzmann scheme for the fluid with standard Langevin
dynamics for the colloidal particle, which is built up from a net of bonded
particles forming the surface of the colloid. The coupling between the two
subsystems is introduced via friction forces. In addition explicit counterions,
also coupled to the fluid, are present. We observe a non-monotonous dependence
of the electrophoretic mobility on the bare colloidal charge. At low surface
charge density we observe a linear increase of the mobility with bare charge,
whereas at higher charges, where more than half of the ions are co-moving with
the colloid, the mobility decreases with increasing bare charge.Comment: 15 pages, 8 figure
Detoxification enzyme activities (CYP1A1 and GST) in the skin of humpback whales as a function of organochlorine burdens and migration status
The activities of glutathione-s-transferase (GST) and cytochrome P-450 1A1 (CYP1A1) enzymes were measured in freshly extracted epidermis of live-biopsied, migrating, southern hemisphere humpback whales (Megaptera novaeangliae). The two quantified enzyme activities did not correlate strongly with each other. Similarly, neither correlated strongly with any of the organochlorine compound groups previously measured in the superficial blubber of the sample biopsy core, likely reflecting the anticipated low levels of typical aryl-hydrocarbon receptor ligands. GST activity did not differ significantly between genders or between northward (early migration) or southward (late migration) migrating cohorts. Indeed, the inter-individual variability in GST measurements was relatively low. This observation raises the possibility that measured activities were basal activities and that GST function was inherently impacted by the fasting state of the sampled animals, as seen in other species. These results do not support the implementation of CYP1A1 or GST as effective biomarkers of organochlorine contaminant burdens in southern hemisphere populations of humpback whales as advocated for other cetacean species. Further investigation of GST activity in feeding versus fasting cohorts may, however, provide some insight into the fasting metabolism of these behaviourally adapted populations. © 2014
A new model for simulating colloidal dynamics
We present a new hybrid lattice-Boltzmann and Langevin molecular dynamics
scheme for simulating the dynamics of suspensions of spherical colloidal
particles. The solvent is modeled on the level of the lattice-Boltzmann method
while the molecular dynamics is done for the solute. The coupling between the
two is implemented through a frictional force acting both on the solvent and on
the solute, which depends on the relative velocity. A spherical colloidal
particle is represented by interaction sites at its surface. We demonstrate
that this scheme quantitatively reproduces the translational and rotational
diffusion of a neutral spherical particle in a liquid and show preliminary
results for a charged spherical particle. We argue that this method is
especially advantageous in the case of charged colloids.Comment: For a movie click on the link below Fig
Kinematics of Metal-Poor Stars in the Galaxy. II. Proper Motions for a Large Non-Kinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected without
kinematic bias, and with available radial velocities, distance estimates, and
metal abundances in the range 0.0 <= [Fe/H] <= -4.0. This update of the Beers
and Sommer-Larsen (1995) catalog includes newly-derived homogeneous photometric
distance estimates, revised radial velocities for a number of stars with
recently obtained high-resolution spectra, and refined metallicities for stars
originally identified in the HK objective-prism survey (which account for
nearly half of the catalog) based on a recent re-calibration. A subset of 1258
stars in this catalog have available proper motions, based on measurements
obtained with the Hipparcos astrometry satellite, or taken from the updated
Astrographic Catalogue (AC 2000; second epoch positions from either the Hubble
Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan
Southern Proper Motion (SPM) Catalog 2.0, and the Lick Northern Proper Motion
(NPM1) Catalog. Our present catalog includes 388 RR Lyrae variables (182 of
which are newly added), 38 variables of other types, and 1680 non-variables,
with distances in the range 0.1 to 40 kpc.Comment: 31 pages, including 8 figures, to appear in AJ (June 2000), full
paper with all figures embedded available at
http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo4
Gravitating Monopole--Antimonopole Chains and Vortex Rings
We construct monopole-antimonopole chain and vortex solutions in
Yang-Mills-Higgs theory coupled to Einstein gravity. The solutions are static,
axially symmetric and asymptotically flat. They are characterized by two
integers (m,n) where m is related to the polar angle and n to the azimuthal
angle. Solutions with n=1 and n=2 correspond to chains of m monopoles and
antimonopoles. Here the Higgs field vanishes at m isolated points along the
symmetry axis. Larger values of n give rise to vortex solutions, where the
Higgs field vanishes on one or more rings, centered around the symmetry axis.
When gravity is coupled to the flat space solutions, a branch of gravitating
monopole-antimonopole chain or vortex solutions arises, and merges at a maximal
value of the coupling constant with a second branch of solutions. This upper
branch has no flat space limit. Instead in the limit of vanishing coupling
constant it either connects to a Bartnik-McKinnon or generalized
Bartnik-McKinnon solution, or, for m>4, n>4, it connects to a new
Einstein-Yang-Mills solution. In this latter case further branches of solutions
appear. For small values of the coupling constant on the upper branches, the
solutions correspond to composite systems, consisting of a scaled inner
Einstein-Yang-Mills solution and an outer Yang-Mills-Higgs solution.Comment: 18 pages, 12 figures, uses revte
Small Satellite Reliability Initiative (SSRI) Knowledge Base Tool: Use Case Review and Future Functionality and Content Direction
The Small Satellite Reliability Initiative (SSRI) Knowledge Base is a comprehensive and searchable online tool that consolidates and organizes resources, best practices, and lessons learned from previous small satellite missions sponsored by NASA, other government agencies, and academia. This free, publicly available tool is available to the entire SmallSat Community.
The SSRI Knowledge Base provides vetted, high-quality sources of information on elements that are key to successful small satellite missions. These resources include SSRI working group generated documents and presentations in addition to existing guides, publications, standards, software tools, websites, and books. The Knowledge Base is fully searchable, offers downloadable content when possible, and otherwise links to or references content directly from within the tool.
This presentation and paper will discuss the motivation for the SSRI Knowledge Base, review educational use cases, and outline plans for further development. The SSRI is a collaborative activity with broad participation from civil, U.S. Department of Defense, and both national and international commercial space systems providers and stakeholders. NASA’s Small Spacecraft Systems Virtual Institute (S3VI) funds the SSRI Knowledge Base. The S3VI is jointly sponsored by NASA’s Space Technology Mission Directorate and Science Mission Directorate
Coulomb Interactions via Local Dynamics: A Molecular--Dynamics Algorithm
We derive and describe in detail a recently proposed method for obtaining
Coulomb interactions as the potential of mean force between charges which are
dynamically coupled to a local electromagnetic field. We focus on the Molecular
Dynamics version of the method and show that it is intimately related to the
Car--Parrinello approach, while being equivalent to solving Maxwell's equations
with freely adjustable speed of light. Unphysical self--energies arise as a
result of the lattice interpolation of charges, and are corrected by a
subtraction scheme based on the exact lattice Green's function. The method can
be straightforwardly parallelized using standard domain decomposition. Some
preliminary benchmark results are presented.Comment: 8 figure
Multi-patch methods in general relativistic astrophysics - I. Hydrodynamical flows on fixed backgrounds
Many systems of interest in general relativistic astrophysics, including
neutron stars, accreting compact objects in X-ray binaries and active galactic
nuclei, core collapse, and collapsars, are assumed to be approximately
spherically symmetric or axisymmetric. In Newtonian or fixed-background
relativistic approximations it is common practice to use spherical polar
coordinates for computational grids; however, these coordinates have
singularities and are difficult to use in fully relativistic models. We
present, in this series of papers, a numerical technique which is able to use
effectively spherical grids by employing multiple patches. We provide detailed
instructions on how to implement such a scheme, and present a number of code
tests for the fixed background case, including an accretion torus around a
black hole.Comment: 26 pages, 20 figures. A high-resolution version is available at
http://www.cct.lsu.edu/~bzink/papers/multipatch_1.pd
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