12,276 research outputs found
Riccati equations and normalized coprime factorizations for strongly stabilizable infinite-dimensional systems
The first part of the paper concerns the existence of strongly stabilizing solutions to the standard algebraic Riccati equation for a class of infinite-dimensional systems of the form Σ(A,B,S−1/2B*,D), where A is dissipative and all the other operators are bounded. These systems are not exponentially stabilizable and so the standard theory is not applicable. The second part uses the Riccati equation results to give formulas for normalized coprime factorizations over H∞ for positive real transfer functions of the form D+S−1/2B*(author−A)−1,B
Lateral Expansion of the Bridges of Cygnus A and other Powerful Radio Sources
Measurements of the width of the radio bridge at several locations along the
bridge for each of four powerful extended radio sources are presented. Adopting
a few simple assumptions, these measurements may be used to predict the radio
surface brightness as a function of position across the radio bridge. The
predicted and observed surface brightnesses across the bridges are compared and
found to agree fairly well. The results are consistent with a simple picture in
which the radio power and size of the radio lobe at the forward edge of the
radio bridge are roughly time-independent for a given source, and the expansion
of the bridge in the lateral direction is adiabatic. There is no indication
that reacceleration or energy transport is important in the bridges of these
sources. The rate of lateral expansion of the bridge just behind the radio lobe
and hotspot in terms of the rate of forward propagation is compared with that
predicted, and found to be in good agreement with the predicted value.Comment: 7 pages, uuencoded compressed postscript. To appear in the Proceding
of the Cygnus A workshop, May 1-4, Green Bank, W
Crater production on Venus and Earth by asteroid and comet impact
New calculations of the collision probabilities of asteroids and comets with Venus were carried out based on the orbits of the known Venus-crossing asteroids and comets. For comparison, asteroid and comet collision probabilities and cratering rates on the Earth and Moon were recalculated and the estimated cratering rates on Venus were normalized to those of the Earth
Results of the 1986 NASA/FAA/DFVLR main rotor test entry in the German-Dutch wind tunnel (DNW)
An acoustics test of a 40%-scale MBB BO-105 helicopter main rotor was conducted in the Deutsch-Niederlandischer Windkanal (DNW). The research, directed by NASA Langley Research Center, concentrated on the generation and radiation of broadband noise and impulsive blade-vortex interaction (BVI) noise over ranges of pertinent rotor operational envelopes. Both the broadband and BVI experimental phases are reviewed, along with highlights of major technical results. For the broadband portion, significant advancement is the demonstration of the accuracy of prediction methods being developed for broadband self noise, due to boundary layer turbulence. Another key result is the discovery of rotor blade-wake interaction (BWI) as an important contributor to mid frequency noise. Also the DNW data are used to determine for full scale helicopters the relative importance of the different discrete and broadband noise sources. For the BVI test portion, a comprehensive data base documents the BVI impulsive noise character and directionality as functions of rotor flight conditions. The directional mapping of BVI noise emitted from the advancing side as well as the retreating side of the rotor constitutes a major advancement in the understanding of this dominant discrete mechanism
Following the relaxation dynamics of photoexcited aniline in the 273-266 nm region using time-resolved photoelectron imaging
Scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES) have been used to investigate the growth morphology of ultra-thin Pb films on the Ni3Al(111) face at room temperature. A previous study [K. Miśków and A. Krupski Appl Surf Sci 273, 2013, 554] using low-energy electron diffraction (LEED) and real time Auger intensity recording has demonstrated that an initial two-dimensional growth of the first Pb monolayer thick ‘wetting layer’ takes place. With further deposition and for T = 300 K, flat three atomic-layer-high islands are grown. Above 350 K, the Stranski–Krastanov growth mode was observed. In the current study, the analysis of STM measurements indicate and confirm that for coverage θ = 1.0 ML two-dimensional growth of the first Pb monolayer took place. Above θ > 1.0 ML, a three-dimensional growth of the Pb islands was observed with a strongly preferred atomic-scale ‘magic height (N),’ hexagonal shape and flat-tops. At coverage θ = 3.5 ML, only islands containing N = 3, 5, 7 and 11 atomic layers of Pb are observed. At the higher coverage θ = 5.5 ML, three types of regular hexagonal islands with side lengths of 25, 30 and 45 nm are observed. Furthermore, three different island adsorption configurations rotated by 10° ± 1° and 30° ± 6° with respect to each other were observed. After an annealing at T = 400 K of 5.5 ML of lead deposited at RT on the Ni3Al(111) the morphology of the surface changes. Post-anneal, islands of Pb are observed above the ‘wetting layer’ with an estimated average size and diameter of 768 ± 291 nm2 and 38.17 ± 6.56 nm and constant uniform height of two atomic layers (N = 2)
Unitarity Cuts with Massive Propagators and Algebraic Expressions for Coefficients
In the first part of this paper, we extend the d-dimensional unitarity cut
method of hep-ph/0609191 to cases with massive propagators. We present formulas
for integral reduction with which one can obtain coefficients of all pentagon,
box, triangle and massive bubble integrals. In the second part of this paper,
we present a detailed study of the phase space integration for unitarity cuts.
We carry out spinor integration in generality and give algebraic expressions
for coefficients, intended for automated evaluation.Comment: 33 pages. v2: notation modified. v3: typos fixe
Low-energy photoelectron transmission through aerosol overlayers
The transmission of low-energy (<1.8eV) photoelectrons through the shell of
core-shell aerosol particles is studied for liquid squalane, squalene, and DEHS
shells. The photoelectrons are exclusively formed in the core of the particles
by two-photon ionization. The total photoelectron yield recorded as a function
of shell thickness (1-80nm) shows a bi-exponential attenuation. For all
substances, the damping parameter for shell thicknesses below 15nm lies between
8 and 9nm, and is tentatively assigned to the electron attenuation length at
electron kinetic energies of ~0.5-1eV. The significantly larger damping
parameters for thick shells (> 20nm) are presumably a consequence of distorted
core-shell structures. A first comparison of aerosol and traditional thin film
overlayer methods is provided
Constraining Gravitino Dark Matter with the Cosmic Microwave Background
We consider super-gravity models in which the lightest supersymmetric
particle (LSP) is a stable gravitino. The next-to-lightest supersymmetric
particle (NLSP) freezes out with its thermal relic density and then decays
after sec, injecting high-energy photons into the cosmic
plasma. These photons heat up the electron plasma which then thermalizes with
the cosmic microwave background (CMB) via Compton scattering, bremsstrahlung
and double-Compton scattering. Contrary to previous studies which assume
instantaneous energy injection, we solve the full kinetic equation for the
photon number density with a source term describing the decay of the NLSP. This
source term is based on the requirement that the injected energy be almost
instantaneously redistributed by Compton scattering, hence leading to a
time-dependent chemical potential. We investigate the case of a stau NLSP and
determine the constraints on the gravitino and stau masses from observations of
the CMB spectrum by assuming that all gravitino LSPs come from stau NLSP
decays. Unlike the analytical approximations, we find that there may be a stau
mass below which the constraint from the CMB spectrum vanishes.Comment: 8 pages, 6 figures, revtex4, replaced to match published versio
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