1,359 research outputs found
Standard Clocks, Orbital Precession and the Cosmological Constant
We discuss the influence of the cosmological constant on the gravitomagnetic clock effect and the gravitational time delay of electromagnetic rays. Moreover, we consider the relative motion of a binary system to linear order in the cosmological constant . The general expression for the effect of on pericenter precession is given for arbitrary orbital eccentricity
Geographic bias related to geocoding in epidemiologic studies
BACKGROUND: This article describes geographic bias in GIS analyses with unrepresentative data owing to missing geocodes, using as an example a spatial analysis of prostate cancer incidence among whites and African Americans in Virginia, 1990–1999. Statistical tests for clustering were performed and such clusters mapped. The patterns of missing census tract identifiers for the cases were examined by generalized linear regression models. RESULTS: The county of residency for all cases was known, and 26,338 (74%) of these cases were geocoded successfully to census tracts. Cluster maps showed patterns that appeared markedly different, depending upon whether one used all cases or those geocoded to the census tract. Multivariate regression analysis showed that, in the most rural counties (where the missing data were concentrated), the percent of a county's population over age 64 and with less than a high school education were both independently associated with a higher percent of missing geocodes. CONCLUSION: We found statistically significant pattern differences resulting from spatially non-random differences in geocoding completeness across Virginia. Appropriate interpretation of maps, therefore, requires an understanding of this phenomenon, which we call "cartographic confounding.
The Central Retail Food Market of Cleveland, Ohio
A study of the Central Retail Food Market in Cleveland, Ohio was made in 1947 at the request of Mayor Thomas A. Burke (Marketing and Facilities Research Branch 3 of the Production and Marketing Administration, U.S. Department of Agriculture) to determine the public need for a new market. In the previous year the city had voted favorably on a bond issue of $1,000,000 for the relocation and rebuilding of the market, provided it could be made self-supporting in a reasonable length of time. The Central Market building, built in 1857 and occupied until December 1949, when it was destroyed by fire....The final determination of whether or not a new public retail market should be built in downtown Cleveland to replace Central Market will have to be made by city officials (excerpt from survey summary).https://engagedscholarship.csuohio.edu/clevmembks/1023/thumbnail.jp
Constraints on stellar convection from multi-colour photometry of Delta Scuti stars
In Delta Scuti star models, the calculated amplitude ratios and phase
differences for multi-colour photometry exhibit a strong dependence on
convection. These observables are tools for determination of the spherical
harmonic degree of the excited modes. The dependence on convection enters
through the complex parameter f, which describes bolometric flux perturbation.
We present a method of simultaneous determination of f and spherical harmonic
degree from multi-colour data and apply it to three Delta Scuti stars. The
method indeed works. Determination of the degree appears unique and the
inferred f's are sufficiently accurate to yield a useful constraint on models
of stellar convection. Furthermore, the method helps to refine stellar
parameters, especially if the identified mode is radial.Comment: 9 pages, 12 figures, to appear in Astronomy and Astrophysic
Laser frequency combs for astronomical observations
A direct measurement of the universe's expansion history could be made by
observing in real time the evolution of the cosmological redshift of distant
objects. However, this would require measurements of Doppler velocity drifts of
about 1 centimeter per second per year, and astronomical spectrographs have not
yet been calibrated to this tolerance. We demonstrate the first use of a laser
frequency comb for wavelength calibration of an astronomical telescope. Even
with a simple analysis, absolute calibration is achieved with an equivalent
Doppler precision of approximately 9 meters per second at about 1.5 micrometers
- beyond state-of-the-art accuracy. We show that tracking complex, time-varying
systematic effects in the spectrograph and detector system is a particular
advantage of laser frequency comb calibration. This technique promises an
effective means for modeling and removal of such systematic effects to the
accuracy required by future experiments to see direct evidence of the
universe's putative acceleration.Comment: Science, 5th September 2008. 18 pages, 7 figures (7 JPG files),
including Supporting Online Material. Version with higher resolution figures
available at http://astronomy.swin.edu.au/~mmurphy/pub.htm
The Fringe Detection Laser Metrology for the GRAVITY Interferometer at the VLTI
Interferometric measurements of optical path length differences of stars over
large baselines can deliver extremely accurate astrometric data. The
interferometer GRAVITY will simultaneously measure two objects in the field of
view of the Very Large Telescope Interferometer (VLTI) of the European Southern
Observatory (ESO) and determine their angular separation to a precision of 10
micro arcseconds in only 5 minutes. To perform the astrometric measurement with
such a high accuracy, the differential path length through the VLTI and the
instrument has to be measured (and tracked since Earth's rotation will
permanently change it) by a laser metrology to an even higher level of accuracy
(corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path
techniques are used for nanometer precision measurements, but with these
methods it is difficult to track the full beam size and to follow the light
path up to the primary mirror of the telescope. Here, we present the
preliminary design of a differential path metrology system, developed within
the GRAVITY project. It measures the instrumental differential path over the
full pupil size and up to the entrance pupil location. The differential phase
is measured by detecting the laser fringe pattern both on the telescopes'
secondary mirrors as well as after reflection at the primary mirror. Based on
our proposed design we evaluate the phase measurement accuracy based on a full
budget of possible statistical and systematic errors. We show that this
metrology design fulfills the high precision requirement of GRAVITY.Comment: Proc. SPIE in pres
The Central Retail Food Market of Cleveland, Ohio
A study of the Central Retail Food Market in Cleveland, Ohio was made in 1947 at the request of Mayor Thomas A. Burke (Marketing and Facilities Research Branch 3 of the Production and Marketing Administration, U.S. Department of Agriculture) to determine the public need for a new market. In the previous year the city had voted favorably on a bond issue of $1,000,000 for the relocation and rebuilding of the market, provided it could be made self-supporting in a reasonable length of time. The Central Market building, built in 1857 and occupied until December 1949, when it was destroyed by fire....The final determination of whether or not a new public retail market should be built in downtown Cleveland to replace Central Market will have to be made by city officials (excerpt from survey summary). Original publication date 1951.https://engagedscholarship.csuohio.edu/clevmembks/1023/thumbnail.jp
Determining the Physical Properties of the B Stars I. Methodology and First Results
We describe a new approach to fitting the UV-to-optical spectra of B stars to
model atmospheres and present initial results. Using a sample of lightly
reddened stars, we demonstrate that the Kurucz model atmospheres can produce
excellent fits to either combined low dispersion IUE and optical photometry or
HST FOS spectrophotometry, as long as the following conditions are fulfilled:
1) an extended grid of Kurucz models is employed,
2) the IUE NEWSIPS data are placed on the FOS absolute flux system using the
Massa & Fitzpatrick (1999) transformation, and
3) all of the model parameters and the effects of interstellar extinction are
solved for simultaneously.
When these steps are taken, the temperatures, gravities, abundances and
microturbulence velocities of lightly reddened B0-A0 V stars are determined to
high precision. We also demonstrate that the same procedure can be used to fit
the energy distributions of stars which are reddened by any UV extinction curve
which can be expressed by the Fitzpatrick & Massa (1990) parameterization
scheme.
We present an initial set of results and verify our approach through
comparisons with angular diameter measurements and the parameters derived for
an eclipsing B star binary. We demonstrate that the metallicity derived from
the ATLAS 9 fits to main sequence B stars is essentially the Fe abundance. We
find that a near zero microturbulence velocity provides the best-fit to all but
the hottest or most luminous stars (where it may become a surrogate for
atmospheric expansion), and that the use of white dwarfs to calibrate UV
spectrophotometry is valid.Comment: 17 pages, including 2 pages of Tables and 6 pages of Figures.
Astrophysical Jounral, in pres
Variational Worldline Approximation for the Relativistic Two-Body Bound State in a Scalar Model
We use the worldline representation of field theory together with a
variational approximation to determine the lowest bound state in the scalar
Wick-Cutkosky model where two equal-mass constituents interact via the exchange
of mesons. Self-energy and vertex corrections are included approximately in a
consistent way as well as crossed diagrams. Only vacuum-polarization effects of
the heavy particles are neglected. In a path integral description of an
appropriate current-current correlator an effective, retarded action is
obtained by integrating out the meson field. As in the polaron problem we
employ a quadratic trial action with variational functions to describe
retardation and binding effects through multiple meson exchange.The variational
equations for these functions are derived, discussed qualitatively and solved
numerically. We compare our results with the ones from traditional approaches
based on the Bethe-Salpeter equation and find an enhanced binding contrary to
some claims in the literature. For weak coupling this is worked out
analytically and compared with results from effective field theories. However,
the well-known instability of the model, which usually is ignored, now appears
at smaller coupling constants than in the one-body case and even when
self-energy and vertex corrections are turned off. This induced instability is
investigated analytically and the width of the bound state above the critical
coupling is estimated.Comment: 62 pages, 7 figures, FBS style, published versio
High-precision wavelength calibration of astronomical spectrographs with laser frequency combs
We describe a possible new technique for precise wavelength calibration of
high-resolution astronomical spectrographs using femtosecond-pulsed mode-locked
lasers controlled by stable oscillators such as atomic clocks. Such `frequency
combs' provide a series of narrow modes which are uniformly spaced according to
the laser's pulse repetition rate and whose absolute frequencies are known a
priori with relative precision better than 10^{-12}. Simulations of frequency
comb spectra show that the photon-limited wavelength calibration precision
achievable with existing echelle spectrographs should be ~1 cm/s when
integrated over a 4000A range. Moreover, comb spectra may be used to accurately
characterise distortions of the wavelength scale introduced by the spectrograph
and detector system. The simulations show that frequency combs with pulse
repetition rates of 5-30GHz are required, given the typical resolving power of
existing and possible future echelle spectrographs. Achieving such high
repetition rates, together with the desire to produce all comb modes with
uniform intensity over the entire optical range, represent the only significant
challenges in the design of a practical system. Frequency comb systems may
remove wavelength calibration uncertainties from all practical spectroscopic
experiments, even those combining data from different telescopes over many
decades.Comment: 10 pages, 6 figures, 1 table. Accepted by MNRAS. v2: Fig. 3 augmented
and minor changes to text (including extended title
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