17,951 research outputs found
Performance of an ablator for Space Shuttle inorbit repair in an arc-plasma airstream
An ablator patch material performed well in an arc plasma environment simulating nominal Earth entry conditions for the Space Shuttle. Ablation tests using vacuum molded cones provided data to optimize the formulation of a two part polymer system for application under space conditions. The blunt cones were made using a Teflon mold and a state of the art caulking gun. Char stability of formulations with various amounts of catalyst and diluent were investigated. The char was found to be unstable in formulations with low amounts of catalyst and high amounts of diluent. The best polymer system determined by these tests was evaluated using a half tile patch in a multiple High Temperature Reusable surface Insulation tile model. It was demonstrated that this ablator could be applied in a space environment using a state of the art caulking gun, would maintain the outer mold line of the thermal protection system during entry, and would keep the bond line temperature at the aluminum tile interface below the design limit
Probing Pair-Correlated Fermionic Atoms through Correlations in Atom Shot Noise
Pair-correlated fermionic atoms are created through dissociation of weakly
bound molecules near a magnetic-field Feshbach resonance. We show that
correlations between atoms in different spin states can be detected using the
atom shot noise in absorption images. Furthermore, using time-of-Flight imaging
we have observed atom pair correlations in momentum space
Problems which are well-posed in a generalized sense with applications to the Einstein equations
In the harmonic description of general relativity, the principle part of
Einstein equations reduces to a constrained system of 10 curved space wave
equations for the components of the space-time metric. We use the
pseudo-differential theory of systems which are well-posed in the generalized
sense to establish the well-posedness of constraint preserving boundary
conditions for this system when treated in second order differential form. The
boundary conditions are of a generalized Sommerfeld type that is benevolent for
numerical calculation.Comment: Final version to appear in Classical and Qunatum Gravit
Gowdy waves as a test-bed for constraint-preserving boundary conditions
Gowdy waves, one of the standard 'apples with apples' tests, is proposed as a
test-bed for constraint-preserving boundary conditions in the non-linear
regime. As an illustration, energy-constraint preservation is separately tested
in the Z4 framework. Both algebraic conditions, derived from energy estimates,
and derivative conditions, deduced from the constraint-propagation system, are
considered. The numerical errors at the boundary are of the same order than
those at the interior points.Comment: 5 pages, 1 figure. Contribution to the Spanish Relativity Meeting
200
Reconstruction of Black Hole Metric Perturbations from Weyl Curvature II: The Regge-Wheeler gauge
Perturbation theory of rotating black holes is described in terms of the Weyl
scalars and ; each satisfying the Teukolsky's complex master
wave equation with spin , and respectively representing outgoing and
ingoing radiation. We explicitly construct the metric perturbations out of
these Weyl scalars in the Regge-Wheeler gauge in the nonrotating limit. We
propose a generalization of the Regge-Wheeler gauge for Kerr background in the
Newman-Penrose language, and discuss the approach for building up the perturbed
spacetime of a rotating black hole. We also provide both-way relationships
between waveforms defined in the metric and curvature approaches in the time
domain, also known as the (inverse-) Chandrasekhar transformations, generalized
to include matter.Comment: 22 pages, no figure
On asymptotically flat solutions of Einstein's equations periodic in time II. Spacetimes with scalar-field sources
We extend the work in our earlier article [4] to show that time-periodic,
asymptotically-flat solutions of the Einstein equations analytic at scri, whose
source is one of a range of scalar-field models, are necessarily stationary. We
also show that, for some of these scalar-field sources, in stationary,
asymptotically-flat solutions analytic at scri, the scalar field necessarily
inherits the symmetry. To prove these results we investigate miscellaneous
properties of massless and conformal scalar fields coupled to gravity, in
particular Bondi mass and its loss.Comment: 29 pages, published in Class. Quant. Grav. Replaced. Typos corrected,
version which appeared in Class. Quant.Gra
ClgR regulation of chaperone and protease systems is essential for Mycobacterium tuberculosis parasitism of the macrophage
Chaperone and protease systems play essential roles in cellular homeostasis and have vital functions in controlling the abundance of specific cellular proteins involved in processes such as transcription, replication, metabolism and virulence. Bacteria have evolved accurate regulatory systems to control the expression and function of chaperones and potentially destructive proteases. Here, we have used a combination of transcriptomics, proteomics and targeted mutagenesis to reveal that the clp gene regulator (ClgR) of Mycobacterium tuberculosis activates the transcription of at least ten genes, including four that encode protease systems (ClpP1/C, ClpP2/C, PtrB and HtrA-like protease Rv1043c) and three that encode chaperones (Acr2, ClpB and the chaperonin Rv3269). Thus, M. tuberculosis ClgR controls a larger network of protein homeostatic and regulatory systems than ClgR in any other bacterium studied to date. We demonstrate that ClgR-regulated transcriptional activation of these systems is essential for M. tuberculosis to replicate in macrophages. Furthermore, we observe that this defect is manifest early in infection, as M. tuberculosis lacking ClgR is deficient in the ability to control phagosome pH 1 h post-phagocytosis
Instability patterns in ultrathin nematic films: comparison between theory and experiment
Motivated by recent experimental observations [U. Delabre et al, Langmuir 24,
3998, 2008] we reconsider an instability of ultrathin nematic films, spread on
liquid substrates. Within a continuum elastic theory of liquid crystals, in the
harmonic approximation, we find an analytical expressions for the critical
thickness as well as for the critical wavenumber, characterizing the onset of
instability towards the stripe phase. Comparing theoretical predictions with
experimental observations, we establish the utility of surface-like term such
as an azimuthal anchoring.Comment: 6 pages, 3 figures, LaTeX macros EPL draft, accepted for publication
in EP
Stable radiation-controlling boundary conditions for the generalized harmonic Einstein equations
This paper is concerned with the initial-boundary value problem for the
Einstein equations in a first-order generalized harmonic formulation. We impose
boundary conditions that preserve the constraints and control the incoming
gravitational radiation by prescribing data for the incoming fields of the Weyl
tensor. High-frequency perturbations about any given spacetime (including a
shift vector with subluminal normal component) are analyzed using the
Fourier-Laplace technique. We show that the system is boundary-stable. In
addition, we develop a criterion that can be used to detect weak instabilities
with polynomial time dependence, and we show that our system does not suffer
from such instabilities. A numerical robust stability test supports our claim
that the initial-boundary value problem is most likely to be well-posed even if
nonzero initial and source data are included.Comment: 27 pages, 4 figures; more numerical results and references added,
several minor amendments; version accepted for publication in Class. Quantum
Gra
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