17,901 research outputs found
Multiprocessor sparse L/U decomposition with controlled fill-in
Generation of the maximal compatibles of pivot elements for a class of small sparse matrices is studied. The algorithm involves a binary tree search and has a complexity exponential in the order of the matrix. Different strategies for selection of a set of compatible pivots based on the Markowitz criterion are investigated. The competing issues of parallelism and fill-in generation are studied and results are provided. A technque for obtaining an ordered compatible set directly from the ordered incompatible table is given. This technique generates a set of compatible pivots with the property of generating few fills. A new hueristic algorithm is then proposed that combines the idea of an ordered compatible set with a limited binary tree search to generate several sets of compatible pivots in linear time. Finally, an elimination set to reduce the matrix is selected. Parameters are suggested to obtain a balance between parallelism and fill-ins. Results of applying the proposed algorithms on several large application matrices are presented and analyzed
Unified constitutive model for single crystal deformation behavior with applications
Single crystal materials are being used in gas turbine airfoils and are candidates for other hot section components because of their increased temperature capabilities and resistance to thermal fatigue. Development of a constitutive model which assesses the inelastic behavior of these materials has been studied in 2 NASA programs: Life Prediction and Constitutive Models for Engine Hot Section Anisotropic Materials and Biaxial Constitutive Equation Development for Single Crystals. The model has been fit to a large body of constitutive data for single crystal PWA 1480 material. The model uses a unified approach for computing total inelastic strains (creep plus plasticity) on crystallographic slip systems reproducing observed directional and strain rate effects as a natural consequence of the summed slip system quantities. The model includes several of the effects that have been reported to influence deformation in single crystal materials, such as shear stress, latent hardening, and cross slip. The model is operational in a commercial Finite Element code and is being installed in a Boundary Element Method code
Spectroscopic and photometric studies of white dwarfs in the Hyades
The Hyades cluster is known to harbour ten so-called classical white dwarf
members. Numerous studies through the years have predicted that more than twice
this amount of degenerate stars should be associated with the cluster. Using
the PPMXL catalog of proper motions and positions, a recent study proposed 17
new white dwarf candidates. We review the membership of these candidates by
using published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to the ten
classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades
origin and three more objects that have an uncertain membership status due to
their unknown or imprecise atmospheric parameters. Among those, two to three
are expected as field stars contamination. Accurate radial velocity
measurements will confirm or reject the candidates. One consequence is that the
longstanding problem that no white dwarf older than ~340 Myr appears to be
associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and
Astrophysics journa
Distributional energy momentum tensor of the extended Kerr geometry
We generalize previous work on the energy-momentum tensor-distribution of the
Kerr geometry by extending the manifold structure into the negative mass
region. Since the extension of the flat part of the Kerr-Schild decomposition
from one sheet to the double cover develops a singularity at the branch surface
we have to take its non-smoothness into account. It is however possible to find
a geometry within the generalized Kerr-Schild class that is in the
Colombeau-sense associated to the maximally analytic Kerr-metric.Comment: 12 pages, latex2e, amslatex and epsf macro
Coulomb Drag between One-Dimensional Wigner Crystal Rings
We consider the Coulomb drag between two metal rings in which the long range
Coulomb interaction leads to the formation of a Wigner crystal. The first ring
is threaded by an Ahranov Bohm flux creating a persistent current J_0. The
second ring is brought in close proximity to the second and due to the Coulomb
interaction between the two rings a drag current J_D is produced in the second.
We investigate this system at zero temperature for perfect rings as well as the
effects of impurities. We show that the Wigner crystal state can in principle
lead to a higher ratio of drag current to drive current J_D/J_0 than in weakly
interacting electron systems.Comment: 12 pages, 10 figure
Synthesis of calculational methods for design and analysis of radiation shields for nuclear rocket systems
Eight computer programs make up a nine volume synthesis containing two design methods for nuclear rocket radiation shields. The first design method is appropriate for parametric and preliminary studies, while the second accomplishes the verification of a final nuclear rocket reactor design
Discovery of kilogauss magnetic fields in three DA white dwarfs
We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD
0446790, WD 1105048, WD 2359434) out of a sample of 12 normal DA white
dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope
equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first
positive detections of magnetic fields in white dwarfs below 30 kG. Although
suspected, it was not clear whether a significant fraction of white dwarfs
contain magnetic fields at this level. These fields may be explained as fossil
relics from magnetic fields in the main-sequence progenitors considerably
enhanced by magnetic flux conservation during the shrinkage of the core. A
detection rate of 25 % (3/12) may indicate now for the first time that a
substantial fraction of white dwarfs have a weak magnetic field. This result,
if confirmed by future observations, would form a cornerstone for our
understanding on the evolution of stellar magnetic fields.
Keywords: stars: white dwarfs - stars: magnetic fields - stars: individual:
WD0446-790, WD1105-048, WD2359-434Comment: 15 pages, 7 figures, Astronomy and Astrophysics, in pres
NFAT5 genes are part of the osmotic regulatory system in Atlantic salmon (Salmo salar)
Acknowledgements This study was supported by a grant from the Biotechnology and Biological Sciences Research Council (BBSRC, BB/H008063/1), UK to DGH and SAM. Funding also came from Research Council Norway for project number 241016 for DGH and EJ. This work was carried out as part of a PhD thesis funded by the Marine Alliance of Science and Technology Scotland (MASTS).Peer reviewedPublisher PD
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with
magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000
K and 50000 K. Through a least-squares minimization scheme with an evolutionary
algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95
previously published plus 14 newly discovered within SDSS, and five discovered
by SEGUE). Since we were limited to a single spectrum for each object we used
only centered magnetic dipoles or dipoles which were shifted along the magnetic
dipole axis. We also statistically investigated the distribution of
magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
The Nature of the Hard-X-Ray Emitting Symbiotic Star RT Cru
We describe Chandra High-Energy Transmission Grating Spectrometer
observations of RT Cru, the first of a new sub-class of symbiotic stars that
appear to contain white dwarfs (WDs) capable of producing hard X-ray emission
out to greater than 50 keV. The production of such hard X-ray emission from the
objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg)
challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV
X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the
optically thin accretion-disk boundary layers of some dwarf novae. The
parameters of the spectral fit confirm that the compact accretor is a WD, and
they are consistent with the WD being massive. We detect rapid, stochastic
variability from the X-ray emission below 4 keV. The combination of flickering
variability and a cooling-flow spectrum indicates that RT Cru is likely powered
by accretion through a disk. Whereas the cataclysmic variable stars with the
hardest X-ray emission are typically magnetic accretors with X-ray flux
modulated at the WD spin period, we find that the X-ray emission from RT Cru is
not pulsed. RT Cru therefore shows no evidence for magnetically channeled
accretion, consistent with our interpretation that the Chandra spectrum arises
from an accretion-disk boundary layer.Comment: 3 figures, accepted for publication in Ap
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