17,943 research outputs found
Navigation in Curved Space-Time
A covariant and invariant theory of navigation in curved space-time with
respect to electromagnetic beacons is written in terms of J. L. Synge's
two-point invariant world function. Explicit equations are given for navigation
in space-time in the vicinity of the Earth in Schwarzschild coordinates and in
rotating coordinates. The restricted problem of determining an observer's
coordinate time when their spatial position is known is also considered
Heat Kernel Asymptotics on Homogeneous Bundles
We consider Laplacians acting on sections of homogeneous vector bundles over
symmetric spaces. By using an integral representation of the heat semi-group we
find a formal solution for the heat kernel diagonal that gives a generating
function for the whole sequence of heat invariants. We argue that the obtained
formal solution correctly reproduces the exact heat kernel diagonal after a
suitable regularization and analytical continuation.Comment: 29 pages, Proceedings of the 2007 Midwest Geometry Conference in
Honor of Thomas P. Branso
Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds
We present CCD photometry in UBVRI of several thousand Galactic field stars
in four large (>1 degree^2) regions centered on diffuse interstellar dust
clouds, commonly referred to as ``cirrus'' clouds (with optical depth A_V less
than unity). Our goal in studying these stars is to investigate the properties
of the cirrus clouds. A comparison of the observed stellar surface density
between on-cloud and off-cloud regions as a function of apparent magnitude in
each of the five bands effectively yields a measure of the extinction through
each cloud. For two of the cirrus clouds, this method is used to derive UBVRI
star counts-based extinction curves, and U-band counts are used to place
constraints on the cloud distance. The color distribution of stars and their
location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order
to determine the amount of selective extinction (reddening) caused by the
cirrus. The color excesses, A_lambda-A_V, derived from stellar color histogram
offsets for the four clouds, are better fit by a reddening law that rises
steeply towards short wavelengths [R_V==A_V/E(B-V)<=2] than by the standard law
(R_V=3.1). This may be indicative of a higher-than-average abundance of small
dust grains relative to larger grains in diffuse cirrus clouds. The shape of
the counts-based effective extinction curve and a comparison of different
estimates of the dust optical depth (extinction optical depth derived from
background star counts/colors; emission optical depth derived from far infrared
measurements), are used to measure the degree of clumpiness in clouds. The set
of techniques explored in this paper can be readily adapted to the Sloan
Digital Sky Survey data set in order to carry out a systematic, large-scale
study of cirrus clouds.Comment: 22 pages, 13 figures (postscript, gif, jpg). Accepted for publication
in the Astronomical Journal, scheduled for the May 1999 issue. Full
resolution postscript versions of all figures are available at
http://www.ucolick.org/~arpad
Step-wedge cluster-randomised community-based trials: An application to the study of the impact of community health insurance
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.BACKGROUND: We describe a step-wedge cluster-randomised community-based trial which has been conducted since 2003 to accompany the implementation of a community health insurance (CHI) scheme in West Africa. The trial aims at overcoming the paucity of evidence-based information on the impact of CHI. Impact is defined in terms of changes in health service utilisation and household protection against the cost of illness. Our exclusive focus on the description and discussion of the methods is justified by the fact that the study relies on a methodology previously applied in the field of disease control, but never in the field of health financing. METHODS: First, we clarify how clusters were defined both in respect of statistical considerations and of local geographical and socio-cultural concerns. Second, we illustrate how households within clusters were sampled. Third, we expound the data collection process and the survey instruments. Finally, we outline the statistical tools to be applied to estimate the impact of CHI. CONCLUSION: We discuss all design choices both in relation to methodological considerations and to specific ethical and organisational concerns faced in the field. On the basis of the appraisal of our experience, we postulate that conducting relatively sophisticated trials (such as our step-wedge cluster-randomised community-based trial) aimed at generating sound public health evidence, is both feasible and valuable also in low income settings. Our work shows that if accurately designed in conjunction with local health authorities, such trials have the potential to generate sound scientific evidence and do not hinder, but at times even facilitate, the implementation of complex health interventions such as CHI
A close look into the carbon disk at the core of the planetary nebula CPD-568032
We present high spatial resolution observations of the dusty core of the
Planetary Nebula with Wolf-Rayet central star CPD-568032. These observations
were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode
providing a typical 300 mas resolution and in interferometric mode using
UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. The
visible HST images exhibit a complex multilobal geometry dominated by faint
lobes. The farthest structures are located at 7" from the star. The mid-IR
environment of CPD-568032 is dominated by a compact source, barely resolved by
a single UT telescope in a 8.7 micron filter. The infrared core is almost fully
resolved with the three 40-45m projected baselines ranging from -5 to 51 degree
but smooth oscillating fringes at low level have been detected in spectrally
dispersed visibilities. This clear signal is interpreted in terms of a ring
structure which would define the bright inner rim of the equatorial disk.
Geometric models allowed us to derive the main geometrical parameters of the
disk. For instance, a reasonably good fit is reached with an achromatic and
elliptical truncated Gaussian with a radius of 97+/-11 AU, an inclination of
28+/-7 degree and a PA for the major axis at 345+/-7 degree. Furthermore, we
performed some radiative transfer modeling aimed at further constraining the
geometry and mass content of the disk, by taking into account the MIDI
dispersed visibilities, spectra, and the large aperture SED of the source.
These models show that the disk is mostly optically thin in the N band and
highly flared.Comment: Paper accepted in A&
Medium Effects on Binary Collisions with the Delta Resonance
To facilitate the relativistic heavy-ion calculations based on transport
equations, the binary collisions involving a resonance in either the
entrance channel or the exit channel are investigated within a Hamiltonian
formulation of interactions. An averaging procedure is developed to
define a quasi-particle and to express the experimentally measured
cross section in terms of an effective cross section. In contrast to previous works, the main feature of
the present approach is that the mass and the momentum of the produced
's are calculated dynamically from the bare vertex interaction of the model Hamiltonian and are constrained by the
unitarity condition. The procedure is then extended to define the effective
cross sections for the experimentally inaccessible and reactions. The predicted cross
sections are significantly different from what are commonly assumed in
relativistic heavy-ion calculations. The potential in nuclear matter
has been calculated by using a Bruckner-Hartree-Fock approximation. By
including the mean-field effects on the propagation, the effective
cross sections of the , and reactions in nuclear matter are
predicted. It is demonstrated that the density dependence is most dramatic in
the energy region close to the pion production threshold.Comment: 20 pages, RevTe
New method to study stochastic growth equations: a cellular automata perspective
We introduce a new method based on cellular automata dynamics to study
stochastic growth equations. The method defines an interface growth process
which depends on height differences between neighbors. The growth rule assigns
a probability exp for a site to
receive one particle at a time and all the sites are updated
simultaneously. Here and are two parameters and
is a function which depends on height of the site and its neighbors. Its
functional form is specified through discretization of the deterministic part
of the growth equation associated to a given deposition process. In particular,
we apply this method to study two linear equations - the Edwards-Wilkinson (EW)
equation and the Mullins-Herring (MH) equation - and a non-linear one - the
Kardar-Parisi-Zhang (KPZ) equation. Through simulations and statistical
analysis of the height distributions of the profiles, we recover the values for
roughening exponents, which confirm that the processes generated by the method
are indeed in the universality classes of the original growth equations. In
addition, a crossover from Random Deposition to the associated correlated
regime is observed when the parameter is varied.Comment: 6 pages, 7 figure
Quark-Gluon String Model Description of Baryon Production in K^{\pm}N Interactions
The process of baryon production in K p collisions at high energies is
considered in the framework of the Quark-Gluon String Model. The contribution
of the string-junction mechanism to the strange baryon production is analysed.
The results of numerical calculations are in reasonable agreement with the data
on inclusive spectra of p, Lambda, bar{Lambda}, and on the bar{Lambda}/Lambda
asymmetry. The predictions for Xi and Omega baryons are presented.Comment: 19 pages, 7 figure
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