824 research outputs found
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
The spectroscopic parameters of sodium cyanide, NaCN (X 1A'), revisited
The study of the rotational spectrum of NaCN (X A') has recently been
extended in frequency and in quantum numbers. Difficulties have been
encountered in fitting the transition frequencies within experimental
uncertainties. Various trial fits traced the difficulties to the incomplete
diagonalization of the Hamiltonian. Employing fewer spectroscopic parameters
than before, the transition frequencies could be reproduced within experimental
uncertainties on average. Predictions of -type -branch transitions with
up to 570 GHz should be reliable to better than 1 MHz. In addition,
modified spectroscopic parameters have been derived for the 13C isotopic
species of NaCN.Comment: 5 pages, no figure, J. Mol. Spectrosc., appeared; CDMS links update
The Tensor to Scalar Ratio of Phantom Dark Energy Models
We investigate the anisotropies in the cosmic microwave background in a class
of models which possess a positive cosmic energy density but negative pressure,
with a constant equation of state w = p/rho < -1. We calculate the temperature
and polarization anisotropy spectra for both scalar and tensor perturbations by
modifying the publicly available code CMBfast. For a constant initial curvature
perturbation or tensor normalization, we have calculated the final anisotropy
spectra as a function of the dark energy density and equation of state w and of
the scalar and tensor spectral indices. This allows us to calculate the
dependence of the tensor-to-scalar ratio on w in a model with phantom dark
energy, which may be important for interpreting any future detection of
long-wavelength gravitational waves.Comment: 5 pages, 4 figure
Cooperation and Self-Regulation in a Model of Agents Playing Different Games
A simple model for cooperation between "selfish" agents, which play an
extended version of the Prisoner's Dilemma(PD) game, in which they use
arbitrary payoffs, is presented and studied. A continuous variable,
representing the probability of cooperation, [0,1], is assigned to
each agent at time . At each time step a pair of agents, chosen at
random, interact by playing the game. The players update their using a
criteria based on the comparison of their utilities with the simplest estimate
for expected income. The agents have no memory and use strategies not based on
direct reciprocity nor 'tags'. Depending on the payoff matrix, the systems
self-organizes - after a transient - into stationary states characterized by
their average probability of cooperation and average equilibrium
per-capita-income . It turns out that the model
exhibit some results that contradict the intuition. In particular, some games
which - {\it a priory}- seems to favor defection most, may produce a relatively
high degree of cooperation. Conversely, other games, which one would bet that
lead to maximum cooperation, indeed are not the optimal for producing
cooperation.Comment: 11 pages, 3 figures, keybords: Complex adaptive systems, Agent-based
models, Social system
Renal pericytes: regulators of medullary blood flow
Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla
Urinary Deoxynivalenol Is Correlated with Cereal Intake in Individuals from the United Kingdom
Background
Deoxynivalenol (DON) is a toxic fungal metabolite that frequently contaminates cereal crops. DON is toxic to animals, but the effects on humans are poorly understood, in part because exposure estimates are of limited precision.
Objectives
In this study we used the U.K. adult National Diet and Nutrition Survey to compare 24-hr urinary DON excretion with cereal intake.
Methods
One hundred subjects were identified for each of the following cereal consumption groups: low (mean, 107 g cereal/day; range, 88–125), medium (mean, 179 g/day; range, 162–195) and high (mean, 300 g/day; range, 276–325). DON was analyzed in 24-hr urine samples by liquid chromatography–mass spectrometry after purification on immunoaffinity columns.
Results
DON was detected in 296 of 300 (98.7%) urine samples. Cereal intake was significantly associated with urinary DON (p < 0.0005), with the geometric mean urinary levels being 6.55 μg DON/day [95% confidence interval (CI), 5.71–7.53]; 9.63 μg/day (95% CI, 8.39–11.05); and 13.24 μg/day (95% CI, 11.54–15.19) for low-, medium-, and high-intake groups, respectively. In multivariable analysis, wholemeal bread (p < 0.0005), white bread (p < 0.0005), “other” bread (p < 0.0005), buns/cakes (p = 0.003), high-fiber breakfast cereal (p = 0.016), and pasta (p = 0.017) were significantly associated with urinary DON. Wholemeal bread was associated with the greatest percent increase in urinary DON per unit of consumption, but white bread contributed approximately twice as much as wholemeal bread to the urinary DON levels because it was consumed in higher amounts.
Conclusion
The majority of adults in the United Kingdom appear to be exposed to DON, and on the basis of the urinary levels, we estimate that some individuals may exceed the European Union (EU) recommended maximum tolerable daily intake of 1,000 ng DON/kg (bw). This exposure biomarker will be a valuable tool for biomonitoring as part of surveillance strategies and in etiologic studies of DON and human disease risk
A model of the Universe including Dark Energy accounted for by both a Quintessence Field and a (negative) Cosmological Constant
In this work we present a model of the universe in which dark energy is
modelled explicitely with both a dynamical quintessence field and a
cosmological constant. Our results confirm the possibility of a future
collapsing universe (for a given region of the parameter space), which is
necessary for a consistent formulation of string theory and quantum field
theory. We have also reproduced the measurements of modulus distance from
supernovae with good accuracy.Comment: 11 pages, 4 figures, only the results for the single exponential
potential are preserved. One author added. Some changes in the reference
section. Submitted to Physical Review
Axion-photon Couplings in Invisible Axion Models
We reexamine the axion-photon couplings in various invisible axion models
motivated by the recent proposal of using optical interferometry at the ASST
facility in the SSCL to search for axion. We illustrate that the assignment of
charges for the fermion fields plays an important role in
determining the couplings. Several simple non-minimal invisible axion models
with suppressed and enhanced axion-photon couplings are constructed,
respectively. We also discuss the implications of possible new experiments to
detect solar axions by conversion to -rays in a static magnetic apparatus
tracking the sun.Comment: 14 pages, LaTeX fil
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Limits on the gravity wave contribution to microwave anisotropies
We present limits on the fraction of large angle microwave anisotropies which
could come from tensor perturbations. We use the COBE results as well as
smaller scale CMB observations, measurements of galaxy correlations, abundances
of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to
provide conservative limits on the tensor-to-scalar ratio for standard
inflationary models. For power-law inflation, for example, we find T/S<0.52 at
95% confidence, with a similar constraint for phi^p potentials. However, for
models with tensor amplitude unrelated to the scalar spectral index it is still
currently possible to have T/S>1.Comment: 23 pages, 7 figures, accepted for publication in Phys. Rev. D.
Calculations extended to blue spectral index, Fig. 6 added, discussion of
results expande
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