28,529 research outputs found
Thermal effects on lattice strain in hcp Fe under pressure
We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron
at high pressures using both first-principles linear response quasiharmonic
calculations based on the full potential linear-muffin-tin-orbital (LMTO)
method and the particle-in-cell (PIC) model for the vibrational partition
function using a tight-binding total-energy method. The tight-binding model
shows excellent agreement with the all-electron LMTO method. When hcp structure
is stable, the calculated geometric mean frequency and Helmholtz free energy of
hcp Fe from PIC and linear response lattice dynamics agree very well, as does
the axial ratio as a function of temperature and pressure. On-site
anharmonicity proves to be small up to the melting temperature, and PIC gives a
good estimate of its sign and magnitude. At low pressures, hcp Fe becomes
dynamically unstable at large c/a ratios, and the PIC model might fail where
the structure approaches lattice instability. The PIC approximation describes
well the vibrational behavior away from the instability, and thus is a
reasonable approach to compute high temperature properties of materials. Our
results show significant differences from earlier PIC studies, which gave much
larger axial ratio increases with increasing temperature, or reported large
differences between PIC and lattice dynamics results.Comment: 9 figure
Probing Fine-Scale Ionospheric Structure with the Very Large Array Radio Telescope
High resolution (~1 arcminute) astronomical imaging at low frequency (below
150 MHz) has only recently become practical with the development of new
calibration algorithms for removing ionospheric distortions. In addition to
opening a new window in observational astronomy, the process of calibrating the
ionospheric distortions also probes ionospheric structure in an unprecedented
way. Here we explore one aspect of this new type of ionospheric measurement,
the differential refraction of celestial source pairs as a function of their
angular separation. This measurement probes variations in the spatial gradient
of the line-of-sight total electron content (TEC) to 0.001 TECU/km accuracy
over spatial scales of under 10 km to over 100 km. We use data from the VLA
Low-frequency Sky Survey (VLSS; Cohen et al. 2007, AJ 134, 1245), a nearly
complete 74 MHz survey of the entire sky visible to the Very Large Array (VLA)
telescope in Socorro, New Mexico. These data comprise over 500 hours of
observations, all calibrated in a standard way. While ionospheric spatial
structure varies greatly from one observation to the next, when analyzed over
hundreds of hours, statistical patterns become apparent. We present a detailed
characterization of how the median differential refraction depends on source
pair separation, elevation and time of day. We find that elevation effects are
large, but geometrically predictable and can be "removed" analytically using a
"thin-shell" model of the ionosphere. We find significantly greater ionospheric
spatial variations during the day than at night. These diurnal variations
appear to affect the larger angular scales to a greater degree indicating that
they come from disturbances on relatively larger spatial scales (100s of km,
rather than 10s of km).Comment: Accepted for publication by The Astronomical Journa
Inhibition of food intake in obese subjects by peptide YY3-36
Background: The gut hormone fragment peptide YY3-36 (PYY) reduces appetite and food intake when infused into subjects of normal weight. In common with the adipocyte hormone leptin, PYY reduces food intake by modulating appetite circuits in the hypothalamus. However, in obesity there is a marked resistance to the action of leptin, which greatly limits its therapeutic effectiveness. We investigated whether obese subjects were also resistant to the anorectic effects of PYY.Methods: We compared the effects of PYY infusion on appetite and food intake in 12 obese and 12 lean subjects in a double-blind, placebo-controlled, crossover study. The plasma levels of PYY, ghrelin, leptin, and insulin were also determined.Results: Caloric intake during a buffet lunch offered two hours after the infusion of PYY was decreased by 30 percent in the obese subjects (P<0.001) and 31 percent in the lean subjects (P<0.001). PYY infusion also caused a significant decrease in the cumulative 24-hour caloric intake in both obese and lean subjects. PYY infusion reduced plasma levels of the appetite-stimulatory hormone ghrelin. Endogenous fasting and postprandial levels of PYY were significantly lower in obese subjects (the mean [+/-SE] fasting PYY levels were 10.2+/-0.7 pmol per liter in the obese group and 16.9+/-0.8 pmol per liter in the lean group, P<0.001). Furthermore, the fasting PYY levels correlated negatively with the body-mass index (r=-0.84, P<0.001).Conclusions: We found that obese subjects were not resistant to the anorectic effects of PYY. Endogenous PYY levels were low in the obese subjects, suggesting that PYY deficiency may contribute to the pathogenesis of obesity
Review: The Potential of Google+ as a Media Literacy Tool
Civic engagement is rarely the initial intent of a social media user. According to a 2011 Pew Internet Life study, nearly two-thirds of social media users are online to keep in touch with friends and family while only a very small percentage (near 5%) utilize it for learning.1 The results of these studies have inspired media literacy scholars and educators to empower social media users to approach the online tools with a mind toward information sharing. The potential in social media is limitless, but many users have to be made aware of the possibilities. Educators in particular should informed of the civic functions Google+ offers the user
Automated identification of Fos expression
The concentration of Fos, a protein encoded by the immediate-early gene c-fos, provides a measure of synaptic activity that may not parallel the electrical activity of neurons. Such a measure is important for the difficult problem of identifying dynamic properties of neuronal circuitries activated by a variety of stimuli and behaviours. We employ two-stage statistical pattern recognition to identify cellular nuclei that express Fos in two-dimensional sections of rat forebrain after administration of antipsychotic drugs. In stage one, we distinguish dark-stained candidate nuclei from image background by a thresholding algorithm and record size and shape measurements of these objects. In stage two, we compare performance of linear and quadratic discriminants, nearest-neighbour and artificial neural network classifiers that employ functions of these measurements to label candidate objects as either Fos nuclei, two touching Fos nuclei or irrelevant background material. New images of neighbouring brain tissue serve as test sets to assess generalizability of the best derived classification rule, as determined by lowest cross-validation misclassification rate. Three experts, two internal and one external, compare manual and automated results for accuracy assessment. Analyses of a subset of images on two separate occasions provide quantitative measures of inter- and intra-expert consistency. We conclude that our automated procedure yields results that compare favourably with those of the experts and thus has potential to remove much of the tedium, subjectivity and irreproducibility of current Fos identification methods in digital microscopy
The Interaction of Venus-like, M-dwarf Planets with the Stellar Wind of Their Host Star
We study the interaction between the atmospheres of Venus-like,
non-magnetized exoplanets orbiting an M-dwarf star, and the stellar wind using
a multi-species Magnetohydrodynaic (MHD) model. We focus our investigation on
the effect of enhanced stellar wind and enhanced EUV flux as the planetary
distance from the star decreases. Our simulations reveal different topologies
of the planetary space environment for sub- and super-Alfvenic stellar wind
conditions, which could lead to dynamic energy deposition in to the atmosphere
during the transition along the planetary orbit. We find that the stellar wind
penetration for non-magnetized planets is very deep, up to a few hundreds of
kilometers. We estimate a lower limit for the atmospheric mass-loss rate and
find that it is insignificant over the lifetime of the planet. However, we
predict that when accounting for atmospheric ion acceleration, a significant
amount of the planetary atmosphere could be eroded over the course of a billion
years.Comment: 13 pages, 7 figures, accepted to Ap
Explicit and Exact Solutions to a Kolmogorov-Petrovskii-Piskunov Equation
Some explicit traveling wave solutions to a Kolmogorov-Petrovskii-Piskunov
equation are presented through two ans\"atze. By a Cole-Hopf transformation,
this Kolmogorov-Petrovskii-Piskunov equation is also written as a bilinear
equation and further two solutions to describe nonlinear interaction of
traveling waves are generated. B\"acklund transformations of the linear form
and some special cases are considered.Comment: 14pages, Latex, to appear in Intern. J. Nonlinear Mechanics, the
original latex file is not complet
High-precision Measurements of Ionospheric TEC Gradients with the Very Large Array VHF System
We have used a relatively long, contiguous VHF observation of a bright cosmic
radio source (Cygnus A) with the Very Large Array (VLA) to demonstrate the
capability of this instrument to study the ionosphere. This interferometer, and
others like it, can observe ionospheric total electron content (TEC)
fluctuations on a much wider range of scales than is possible with many other
instruments. We have shown that with a bright source, the VLA can measure
differential TEC values between pairs of antennas (delta-TEC) with an precision
of 0.0003 TECU. Here, we detail the data reduction and processing techniques
used to achieve this level of precision. In addition, we demonstrate techniques
for exploiting these high-precision delta-TEC measurements to compute the TEC
gradient observed by the array as well as small-scale fluctuations within the
TEC gradient surface. A companion paper details specialized spectral analysis
techniques used to characterize the properties of wave-like fluctuations within
this data.Comment: accepted for publication in Radio Scienc
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