4,025 research outputs found
Conceptual design studies for large free-flying solar-reflector spacecraft
The 1 km diameter reflecting film surface is supported by a lightweight structure which may be automatically deployed after launch in the Space Shuttle. A twin rotor, control moment gyroscope, with deployable rotors, is included as a primary control actuator. The vehicle has a total specific mass of less than 12 g/sq m including allowances for all required subsystems. The structural elements were sized to accommodate the loads of a typical SOLARES type mission where a swam of these free flying satellites is employed to concentrate sunlight on a number of energy conversion stations on the ground
A New Measurement of Cosmic Ray Composition at the Knee
The Dual Imaging Cerenkov Experiment (DICE) was designed and operated for
making elemental composition measurements of cosmic rays near the knee of the
spectrum at several PeV. Here we present the first results using this
experiment from the measurement of the average location of the depth of shower
maximum, , in the atmosphere as a function of particle energy. The value
of near the instrument threshold of ~0.1 PeV is consistent with
expectations from previous direct measurements. At higher energies there is
little change in composition up to ~5 PeV. Above this energy is deeper
than expected for a constant elemental composition implying the overall
elemental composition is becoming lighter above the knee region. These results
disagree with the idea that cosmic rays should become on average heavier above
the knee. Instead they suggest a transition to a qualitatively different
population of particles above 5 PeV.Comment: 7 pages, LaTeX, two eps figures, aas2pp4.sty and epsf.sty included,
accepted by Ap.J. Let
Calculations for Mirror Symmetry with D-branes
We study normal functions capturing D-brane superpotentials on several one-
and two-parameter Calabi-Yau hypersurfaces and complete intersections in
weighted projective space. We calculate in the B-model and interpret the
results using mirror symmetry in the large volume regime, albeit without
identifying the precise A-model geometry in all cases. We identify new classes
of extensions of Picard-Fuchs equations, as well as a novel type of topology
changing phase transition involving quantum D-branes. A 4-d domain wall which
is obtained in one region of closed string moduli space from wrapping a
four-chain interpolating between two Lagrangian submanifolds is, for other
values of the parameters, represented by a disk ending on a single Lagrangian.Comment: 42 page
Faint HI 21-cm Emission Line Wings at Forbidden-Velocities
We present the results of a search for faint HI 21-cm emission line wings at
velocities forbidden by Galactic rotation in the Galactic plane using the
Leiden/Dwingeloo HI Survey data and the HI Southern Galactic Plane Survey data.
These ``forbidden-velocity wings (FVWs)'' appear as protruding excessive
emission in comparison with their surroundings in limited (< 2 deg) spatial
regions over velocity extent more than ~20 km/s in large-scale (l-v) diagrams.
Their high-velocities imply that there should be some dynamical phenomena
associated. We have identified 87 FVWs. We present their catalog, and discuss
their distribution and statistical properties. We found that 85% of FVWs are
not coincident with known supernova remnants (SNRs), galaxies, or high-velocity
clouds. Their natures are currently unknown. We suspect that many of them are
fast-moving HI shells and filaments associated with the oldest SNRs that are
essentially invisible except via their HI line emission. We discuss other
possible origins.Comment: 41 pages, 14 figures, to be published in apj
Protein dynamics with off-lattice Monte Carlo moves
A Monte Carlo method for dynamics simulation of all-atom protein models is
introduced, to reach long times not accessible to conventional molecular
dynamics. The considered degrees of freedom are the dihedrals at
C-atoms. Two Monte Carlo moves are used: single rotations about
torsion axes, and cooperative rotations in windows of amide planes, changing
the conformation globally and locally, respectively. For local moves Jacobians
are used to obtain an unbiased distribution of dihedrals. A molecular dynamics
energy function adapted to the protein model is employed. A polypeptide is
folded into native-like structures by local but not by global moves.Comment: 10 pages, 4 Postscript figures, uses epsf.sty and a4.sty; scheduled
tentatively for Phys.Rev.E issue of 1 March 199
Kinematics of elliptical galaxies with a diffuse dust component
Observations show that early-type galaxies contain a considerable amount of
interstellar dust, most of which is believed to exist as a diffusely
distributed component. We construct a four-parameter elliptical galaxy model in
order to investigate the effects of such a smooth absorbing component on the
projection of kinematic quantities, such as the line profiles and their
moments. We investigate the dependence on the optical depth and on the dust
geometry. Our calculations show that both the amplitude and the morphology of
these quantities can be significantly affected. Dust effects should therefore
be taken in consideration when interpreting photometric and kinematic
properties, and correlations that utilize these quantities.Comment: 12 pages, 9 figures, accepted for publication in MNRA
The nature of iron-oxygen vacancy defect centers in PbTiO3
The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy
forms a charged defect associate, oriented along the crystallographic c-axis.
Its microscopic structure has been analyzed in detail comparing results from a
semi-empirical Newman superposition model analysis based on finestructure data
and from calculations using density functional theory.
Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor
center. The position of the iron ion in the ferroelectric phase is found to be
similar to the B-site in the paraelectric phase. Partial charge compensation is
locally provided by a directly coordinated oxygen vacancy.
Using high-resolution synchrotron powder diffraction, it was verified that
lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of
1.0721.Comment: 11 pages, 5 figures, accepted in Phys. Rev.
Systematic techniques for assisting recruitment to trials (START): study protocol for embedded, randomized controlled trials
BACKGROUND: Randomized controlled trials play a central role in evidence-based practice, but recruitment of participants, and retention of them once in the trial, is challenging. Moreover, there is a dearth of evidence that research teams can use to inform the development of their recruitment and retention strategies. As with other healthcare initiatives, the fairest test of the effectiveness of a recruitment strategy is a trial comparing alternatives, which for recruitment would mean embedding a recruitment trial within an ongoing host trial. Systematic reviews indicate that such studies are rare. Embedded trials are largely delivered in an ad hoc way, with interventions almost always developed in isolation and tested in the context of a single host trial, limiting their ability to contribute to a body of evidence with regard to a single recruitment intervention and to researchers working in different contexts. METHODS/DESIGN: The Systematic Techniques for Assisting Recruitment to Trials (START) program is funded by the United Kingdom Medical Research Council (MRC) Methodology Research Programme to support the routine adoption of embedded trials to test standardized recruitment interventions across ongoing host trials. To achieve this aim, the program involves three interrelated work packages: (1) methodology - to develop guidelines for the design, analysis and reporting of embedded recruitment studies; (2) interventions - to develop effective and useful recruitment interventions; and (3) implementation - to recruit host trials and test interventions through embedded studies. DISCUSSION: Successful completion of the START program will provide a model for a platform for the wider trials community to use to evaluate recruitment interventions or, potentially, other types of intervention linked to trial conduct. It will also increase the evidence base for two types of recruitment intervention. TRIAL REGISTRATION: The START protocol covers the methodology for embedded trials. Each embedded trial is registered separately or as a substudy of the host trial
Mirror Symmetry for Toric Branes on Compact Hypersurfaces
We use toric geometry to study open string mirror symmetry on compact
Calabi-Yau manifolds. For a mirror pair of toric branes on a mirror pair of
toric hypersurfaces we derive a canonical hypergeometric system of differential
equations, whose solutions determine the open/closed string mirror maps and the
partition functions for spheres and discs. We define a linear sigma model for
the brane geometry and describe a correspondence between dual toric polyhedra
and toric brane geometries. The method is applied to study examples with
obstructed and classically unobstructed brane moduli at various points in the
deformation space. Computing the instanton expansion at large volume in the
flat coordinates on the open/closed deformation space we obtain predictions for
enumerative invariants.Comment: 36 pages, references adde
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