5,773 research outputs found
A light-cone gauge for black-hole perturbation theory
The geometrical meaning of the Eddington-Finkelstein coordinates of
Schwarzschild spacetime is well understood: (i) the advanced-time coordinate v
is constant on incoming light cones that converge toward r=0, (ii) the angles
theta and phi are constant on the null generators of each light cone, (iii) the
radial coordinate r is an affine-parameter distance along each generator, and
(iv) r is an areal radius, in the sense that 4 pi r^2 is the area of each
two-surface (v,r) = constant. The light-cone gauge of black-hole perturbation
theory, which is formulated in this paper, places conditions on a perturbation
of the Schwarzschild metric that ensure that properties (i)--(iii) of the
coordinates are preserved in the perturbed spacetime. Property (iv) is lost in
general, but it is retained in exceptional situations that are identified in
this paper. Unlike other popular choices of gauge, the light-cone gauge
produces a perturbed metric that is expressed in a meaningful coordinate
system; this is a considerable asset that greatly facilitates the task of
extracting physical consequences. We illustrate the use of the light-cone gauge
by calculating the metric of a black hole immersed in a uniform magnetic field.
We construct a three-parameter family of solutions to the perturbative
Einstein-Maxwell equations and argue that it is applicable to a broader range
of physical situations than the exact, two-parameter Schwarzschild-Melvin
family.Comment: 12 page
Study of a soft lander/support module for Mars missions. Volume 3 - Appendixes Final summary report
Soft lander support module for Mars missions - lunar module radar evaluation and vernier phase simulatio
Critical behavior in an evolutionary Ultimatum Game
Experimental studies have shown the ubiquity of altruistic behavior in human
societies. The social structure is a fundamental ingredient to understand the
degree of altruism displayed by the members of a society, in contrast to
individual-based features, like for example age or gender, which have been
shown not to be relevant to determine the level of altruistic behavior. We
explore an evolutionary model aiming to delve how altruistic behavior is
affected by social structure. We investigate the dynamics of interacting
individuals playing the Ultimatum Game with their neighbors given by a social
network of interaction. We show that a population self-organizes in a critical
state where the degree of altruism depends on the topology characterizing the
social structure. In general, individuals offering large shares but in turn
accepting large shares, are removed from the population. In heterogeneous
social networks, individuals offering intermediate shares are strongly selected
in contrast to random homogeneous networks where a broad range of offers, below
a critical one, is similarly present in the population.Comment: 13 pages, 7 figure
Fat transforms ascorbic acid from inhibiting to promoting acid-catalysed N-nitrosation
<b>Background</b>: The major potential site of acid nitrosation is the proximal stomach, an anatomical site prone to
a rising incidence of metaplasia and adenocarcinoma. Nitrite, a pre-carcinogen present in saliva, can be
converted to nitrosating species and N-nitroso compounds by acidification at low gastric pH in the presence
of thiocyanate.
<b>Aims</b>: To assess the effect of lipid and ascorbic acid on the nitrosative chemistry under conditions simulating
the human proximal stomach.
<b>Methods</b>: The nitrosative chemistry was modelled in vitro by measuring the nitrosation of four secondary
amines under conditions simulating the proximal stomach. The N-nitrosamines formed were measured by gas
chromatography–ion-trap tandem mass spectrometry, while nitric oxide and oxygen levels were measured
amperometrically.
<b>Results</b>: In absence of lipid, nitrosative stress was inhibited by ascorbic acid through conversion of nitrosating
species to nitric oxide. Addition of ascorbic acid reduced the amount of N-nitrosodimethylamine formed by
fivefold, N-nitrosomorpholine by .1000-fold, and totally prevented the formation of N-nitrosodiethylamine
and N-nitrosopiperidine. In contrast, when 10% lipid was present, ascorbic acid increased the amount of Nnitrosodimethylamine,
N-nitrosodiethylamine and N-nitrosopiperidine formed by approximately 8-, 60- and
140-fold, respectively, compared with absence of ascorbic acid.
<b>Conclusion</b>: The presence of lipid converts ascorbic acid from inhibiting to promoting acid nitrosation. This
may be explained by nitric oxide, formed by ascorbic acid in the aqueous phase, being able to regenerate
nitrosating species by reacting with oxygen in the lipid phase
Symmetry and Surface Symmetry Energies in Finite Nuclei
A study of properties of the symmetry energy of nuclei is presented based on
density functional theory. Calculations for finite nuclei are given so that the
study includes isospin dependent surface symmetry considerations as well as
isospin independent surface effects. Calculations are done at both zero and
non-zero temperature. It is shown that the surface symmetry energy term is the
most sensitive to the temperature while the bulk energy term is the least
sensitive. It is also shown that the temperature dependence terms are
insensitive to the force used and even more insensitive to the existence of
neutron skin. Results for a symmetry energy with both volume and surface terms
are compared with a symmetry energy with only volume terms along the line of
stability. Differences of several MeV are shown over a good fraction of
the total mass range in . Also given are calculations for the bulk, surface
and Coulomb terms.Comment: 11 pages, 2 figures, Added a new tabl
Common Weeds as a Cause of Perirenal Edema In Swine
The disease problem described here has been diagnosed with increased frequency in Iowa during the last few years. The literature contains very few references to perirenal edema as a lesion in swine, and even fewer references to the condition as a distinct disease syndrome
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IT Alignment with Business Strategies in Healthcare Organizations: An Empirical Analysis
This paper describes a study that assessed the performance implications of aligning information technology (IT) strategy to overall business strategy across a variety of health care organization (HCO) structures. We obtained survey results from senior executives of 178 hospitals to identify key configurations of IT strategic practices, business strategy and HCO structures. Using K- means cluster analysis, we identified which business strategies correlate strongly with certain IT strategy types. Our results indicate that HCOs achieve superior performance through unique combinations of business and IT strategy, suggesting that correctly aligning these strategies is a critical decision for healthcare organizations
A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
We present radial velocities, equivalent widths, model atmosphere parameters,
and abundances or upper limits for 53 species of 48 elements derived from high
resolution optical spectroscopy of 313 metal-poor stars. A majority of these
stars were selected from the metal-poor candidates of the HK Survey of Beers,
Preston, and Shectman. We derive detailed abundances for 61% of these stars for
the first time. Spectra were obtained during a 10-year observing campaign using
the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at
Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan
J. Smith Telescope at McDonald Observatory, and the High Resolution
Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform
a standard LTE abundance analysis using MARCS model atmospheres, and we apply
line-by-line statistical corrections to minimize systematic abundance
differences arising when different sets of lines are available for analysis. We
identify several abundance correlations with effective temperature. A
comparison with previous abundance analyses reveals significant differences in
stellar parameters, which we investigate in detail. Our metallicities are, on
average, lower by approx. 0.25 dex for red giants and approx. 0.04 dex for
subgiants. Our sample contains 19 stars with [Fe/H] < -3.5, 84 stars with
[Fe/H] < -3.0, and 210 stars with [Fe/H] < -2.5. Detailed abundances are
presented here or elsewhere for 91% of the 209 stars with [Fe/H] < -2.5 as
estimated from medium resolution spectroscopy by Beers, Preston, and Shectman.
We will discuss the interpretation of these abundances in subsequent papers.Comment: Accepted for publication in the Astronomical Journal. 60 pages, 59
figures, 18 tables. Machine-readable versions of the long tables can be found
in the ancillary data file
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