17,046 research outputs found
Policy issues and data communications for NASA earth observation missions until 1985
The series of LANDSAT sensors with the highest potential data rates of the missions were examined. An examination of LANDSAT imagery uses shows that relatively few require transmission of the full resolution data on a repetitive quasi real time basis. Accuracy of global crop size forecasting can possibly be improved through information derived from LANDSAT imagery. A current forecasting experiment uses the imagery for crop area estimation only, yield being derived from other data sources
Distribution of the second virial coefficients of globular proteins
George and Wilson [Acta. Cryst. D 50, 361 (1994)] looked at the distribution
of values of the second virial coefficient of globular proteins, under the
conditions at which they crystallise. They found the values to lie within a
fairly narrow range. We have defined a simple model of a generic globular
protein. We then generate a set of proteins by picking values for the
parameters of the model from a probability distribution. At fixed solubility,
this set of proteins is found to have values of the second virial coefficient
that fall within a fairly narrow range. The shape of the probability
distribution of the second virial coefficient is Gaussian because the second
virial coefficient is a sum of contributions from different patches on the
protein surface.Comment: 5 pages, including 3 figure
Structure and giant magnetoresistance of granular Co-Cu nanolayers prepared by cross-beam PLD
A series of Co_xCu_{100-x} (x = 0, 40...75, 100) layers with thicknesses
in-between 13 nm and 55 nm were prepared on silicon substrates using cross-beam
pulsed laser deposition. Wide-angle X-ray diffraction (WAXRD), transmission
electron microscopy (TEM) and electrical transport measurements revealed a
structure consisting of decomposed cobalt and copper grains with grain sizes of
about 10 nm. The influence of cobalt content and layer thickness on the grain
size is discussed. Electron diffraction (ED) indicates the presence of an
intermetallic Co-Cu phase of Cu3Au structure-type. Thermal treatment at
temperatures between 525 K and 750 K results in the progressive decomposition
of Co and Cu, with an increase of the grain sizes up to about 100 nm. This is
tunable by controlling the temperature and duration of the anneal, and is
directly observable in WAXRD patterns and TEM images. A careful analysis of
grain size and the coherence length of the radiation used allows for an
accurate interpretation of the X-ray diffraction patterns, by taking into
account coherent and non-coherent scattering. The alloy films show a giant
magnetoresistance of 1...2.3 % with the maximum obtained after annealing at
around 725 K.Comment: 9 pages, 9 figure
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction
We depict the use of x-ray diffraction as a tool to directly probe the strain
status in rolled-up semiconductor tubes. By employing continuum elasticity
theory and a simple model we are able to simulate quantitatively the strain
relaxation in perfect crystalline III-V semiconductor bi- and multilayers as
well as in rolled-up layers with dislocations. The reduction in the local
elastic energy is evaluated for each case. Limitations of the technique and
theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference
3-Dimensional Core-Collapse
In this paper, we present the results of 3-dimensional collapse simulations
of rotating stars for a range of stellar progenitors. We find that for the
fastest spinning stars, rotation does indeed modify the convection above the
proto-neutron star, but it is not fast enough to cause core fragmentation.
Similarly, although strong magnetic fields can be produced once the
proto-neutron star cools and contracts, the proto-neutron star is not spinning
fast enough to generate strong magnetic fields quickly after collapse and, for
our simulations, magnetic fields will not dominate the supernova explosion
mechanism. Even so, the resulting pulsars for our fastest rotating models may
emit enough energy to dominate the total explosion energy of the supernova.
However, more recent stellar models predict rotation rates that are much too
slow to affect the explosion, but these models are not sophisticated enough to
determine whether the most recent, or past, stellar rotation rates are most
likely. Thus, we must rely upon observational constraints to determine the true
rotation rates of stellar cores just before collapse. We conclude with a
discussion of the possible constraints on stellar rotation which we can derive
from core-collapse supernovae.Comment: 34 pages (5 of 17 figures missing), For full paper, goto
http://qso.lanl.gov/~clf/papers/rot.ps.gz accepted by Ap
Jamming transitions in a schematic model of suspension rheology
We study the steady-state response to applied stress in a simple scalar model
of sheared colloids. Our model is based on a schematic (F2) model of the glass
transition, with a memory term that depends on both stress and shear rate. For
suitable parameters, we find transitions from a fluid to a nonergodic, jammed
state, showing zero flow rate in an interval of applied stress. Although the
jammed state is a glass, we predict that jamming transitions have an analytical
structure distinct from that of the conventional mode coupling glass
transition. The static jamming transition we discuss is also distinct from
hydrodynamic shear thickening.Comment: 7 pages; 3 figures; improved version with added references. Accepted
for publication in Europhysics Letter
Automatic Quantum Error Correction
Criteria are given by which dissipative evolution can transfer populations
and coherences between quantum subspaces, without a loss of coherence. This
results in a form of quantum error correction that is implemented by the joint
evolution of a system and a cold bath. It requires no external intervention
and, in principal, no ancilla. An example of a system that protects a qubit
against spin-flip errors is proposed. It consists of three spin 1/2 magnetic
particles and three modes of a resonator. The qubit is the triple quantum
coherence of the spins, and the photons act as ancilla.Comment: 16 pages 12 fig LaTex uses multicol, graphicx expanded version of
letter submitted to Phys Rev Let
X-ray diffraction peak profiles from threading dislocations in GaN epitaxial films
We analyze the lineshape of x-ray diffraction profiles of GaN epitaxial
layers with large densities of randomly distributed threading dislocations. The
peaks are Gaussian only in the central, most intense part of the peak, while
the tails obey a power law. The decay typical for random dislocations
is observed in double-crystal rocking curves. The entire profile is well fitted
by a restricted random dislocation distribution. The densities of both edge and
screw threading dislocations and the ranges of dislocation correlations are
obtained
Demixing in a single-peak distributed polydisperse mixture of hard spheres
An analytic derivation of the spinodal of a polydisperse mixture is
presented. It holds for fluids whose excess free energy can be accurately
described by a function of a few moments of the size distribution. It is shown
that one such mixture of hard spheres in the Percus-Yevick approximation never
demixes, despite its size distribution. In the
Boublik-Mansoori-Carnahan-Starling-Leland approximation, though, it demixes for
a sufficiently wide log-normal size distribution. The importance of this result
is twofold: first, this distribution is unimodal, and yet it phase separates;
and second, log-normal size distributions appear in many experimental contexts.
The same phenomenon is shown to occur for the fluid of parallel hard cubes.Comment: 4 pages, 2 figures, needs revtex, multicol, epsfig and amstex style
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