18,918 research outputs found
Exact moments in a continuous time random walk with complete memory of its history
We present a continuous time generalization of a random walk with complete
memory of its history [Phys. Rev. E 70, 045101(R) (2004)] and derive exact
expressions for the first four moments of the distribution of displacement when
the number of steps is Poisson distributed. We analyze the asymptotic behavior
of the normalized third and fourth cumulants and identify new transitions in a
parameter regime where the random walk exhibits superdiffusion. These
transitions, which are also present in the discrete time case, arise from the
memory of the process and are not reproduced by Fokker-Planck approximations to
the evolution equation of this random walk.Comment: Revtex4, 10 pages, 2 figures. v2: applications discussed, clarity
improved, corrected scaling of third momen
Biaxial fatigue loading of notched composites
Thin walled, 2.54-cm (1-in.) diameter tubular specimens of T300/934 graphite/epoxy were fabricated and fatigue cycled in combinations of axial, torsional, and internal pressure loading. Two different four-ply layup configurations were tested: (0/90)S and (+ or - 45)S; all tubes contained a 0.48-cm (3/16-in.) diameter circular hole penetrating one wall midway along the tube length. S-N curves were developed to characterize fatigue behavior under pure axial, torsional, or internal pressure loading, as well as combined loading fatigue. A theory was developed based on the Hill plane stress model which enabled the S-N curve for combined stress states to be predicted from the S-N data for the uniaxial loading modes. Correlation of the theory with the experimental data proved to be remarkably good
Contacting the spirits of the dead: paranormal belief and the teenage worldview
A number of previous studies have examined both the overall level of belief expressed by young people in the paranormal and the major demographic predictors of such belief. Building on this research tradition, the present study examines how one specific paranormal belief concerning contact with the spirits of the dead integrates with the wider teenage worldview. Data provided by 33,982 pupils age 13 to 15 years throughout England and Wales demonstrated that almost one in three young people (31%) believed that it is possible to contact the spirits of the dead. Compared with young people who did not share this belief, the young people who believed in the possibility of contacting the spirits of the dead displayed lower psychological wellbeing, higher anxiety, greater isolation, greater alienation, less positive social attitudes, and less socially conforming lifestyles. Overall, paranormal beliefs seem to be associated with a less healthy worldview, in both personal and social terms
Gas-cooling by dust during dynamical fragmentation
We suggest that the abrupt switch, from hierarchical clustering on scales
larger than 0.04 pc, to binary (and occasionally higher multiple) systems on
smaller scales, which Larson has deduced from his analysis of the grouping of
pre-Main-Sequence stars in Taurus, arises because pre-protostellar gas becomes
thermally coupled to dust at sufficiently high densities. The resulting change
from gas-cooling by molecular lines at low densities to gas-cooling by dust at
high densities enables the matter to radiate much more efficiently, and hence
to undergo dynamical fragmentation.
We derive the domain where gas-cooling by dust facilitates dynamical
fragmentation. Low-mass (i.e. solar mass) clumps - those supported mainly by
thermal pressure - can probably access this domain spontaneously, albeit rather
quasistatically, provided they exist in a region where external perturbations
are few and far between. More massive clumps probably require an impulsive
external perturbation, for instance a supersonic collision with another clump,
in order for the gas to reach sufficiently high density to couple thermally to
the dust. Impulsive external perturbations should promote fragmentation, by
generating highly non-line ar substructures which can then be amplified by
gravity during the subsequent collapse.Comment: 9 pages, 4 figures, accepted by MNRA
Can a connectionist model explain the processing of regularly and irregularly inflected words in German as L1 and L2?
The connectionist model is a prevailing model of the structure and functioning of the cognitive system of the processing of morphology. According to this model, the morphology of regularly and irregularly inflected words (e.g., verb participles and noun plurals) is processed in the same cognitive network. A validation of the connectionist model of the processing of morphology in German as L2 has yet to be achieved. To investigate L2-specific aspects, we compared a group of L1 speakers of German with speakers of German as L2. L2 and L1 speakers of German were assigned to their respective group by their reaction times in picture naming prior to the central task. The reaction times in the lexical decision task of verb participles and noun plurals were largely consistent with the assumption of the connectionist model. Interestingly, speakers of German as L2 showed a specific advantage for irregular compared with regular verb participles
The effect of estrogen on intracellular Ca2+ and Na+ regulation in heart failure
Background: Under-representation of women in heart failure (HF) trials and contradictory findings of estrogen supplementation highlight the need to investigate the involvement of estrogen in the progression of heart failure. Objectives: To characterize the influence of estrogen on cardiac function during the onset of heart failure in a controlled animal model that lacks traditional comorbidities. Methods: HF was induced by aortic constriction (AC) in female guinea pigs. Selected AC animals were ovariectomized (ACOV) and a group of these received 17β-estradiol (ACOV+E) supplementation. 150 days post-AC surgery, left ventricular myocytes were isolated and their electrophysiology and Ca2+ and Na+ regulation examined. Results: ACOV animals heart weight/body weight ratios increased by 11% and in vivo fractional shortening decreased by 14% compared with the AC group, suggesting greater impairment of cardiac function in the absence of estrogen. Action potential duration increased in all three HF groups, the extent of the changes being estrogen-dependent. Myocytes from ACOV animals had reduced Ca2+ transient amplitudes, slower transient decay kinetics, decreased SR Ca2+ contents but increased Ca2+ spark frequencies and spark-mediated SR leak compared with the AC and ACOV+E groups. The Na+/K+ ATPase current densities and Na+ extrusion rates were reduced by 13% and 19%, respectively, in parallel with a 17% increase in INa,L current densities in the ACOV group compared with the AC group. Conclusions: Long-term absence of ovarian hormones exacerbates the decline in cardiac function during the progression to HF. Estrogen supplementation reverses these aggravating effects
Double Lobed Radio Quasars from the Sloan Digital Sky Survey
We have combined a sample of 44984 quasars, selected from the Sloan Digital
Sky Survey (SDSS) Data Release 3, with the FIRST radio survey. Using a novel
technique where the optical quasar position is matched to the complete radio
environment within 450", we are able to characterize the radio morphological
make-up of what is essentially an optically selected quasar sample, regardless
of whether the quasar (nucleus) itself has been detected in the radio. About
10% of the quasar population have radio cores brighter than 0.75 mJy at 1.4
GHz, and 1.7% have double lobed FR2-like radio morphologies. About 75% of the
FR2 sources have a radio core (> 0.75 mJy). A significant fraction (~40%) of
the FR2 quasars are bent by more than 10 degrees, indicating either
interactions of the radio plasma with the ICM or IGM. We found no evidence for
correlations with redshift among our FR2 quasars: radio lobe flux densities and
radio source diameters of the quasars have similar distributions at low (mean
0.77) and high (mean 2.09) redshifts. Using a smaller high reliability FR2
sample of 422 quasars and two comparison samples of radio-quiet and non-FR2
radio-loud quasars, matched in their redshift distributions, we constructed
composite optical spectra from the SDSS spectroscopic data. Based on these
spectra we can conclude that the FR2 quasars have stronger high-ionization
emission lines compared to both the radio quiet and non-FR2 radio loud sources.
This is consistent with the notion that the emission lines are brightened by
ongoing shock ionization of ambient gas in the quasar host as the radio source
expands.Comment: 20 pages, 10 figures - some of which have been reduced in quality /
size. Accepted for publication in the Astronomical Journa
Evaluation of Skylab (EREP) data for forest and rangeland surveys
The author has identified the following significant results. Four widely separated sites (near Augusta, Georgia; Lead, South Dakota; Manitou, Colorado; and Redding, California) were selected as typical sites for forest inventory, forest stress, rangeland inventory, and atmospheric and solar measurements, respectively. Results indicated that Skylab S190B color photography is good for classification of Level 1 forest and nonforest land (90 to 95 percent correct) and could be used as a data base for sampling by small and medium scale photography using regression techniques. The accuracy of Level 2 forest and nonforest classes, however, varied from fair to poor. Results of plant community classification tests indicate that both visual and microdensitometric techniques can separate deciduous, conifirous, and grassland classes to the region level in the Ecoclass hierarchical classification system. There was no consistency in classifying tree categories at the series level by visual photointerpretation. The relationship between ground measurements and large scale photo measurements of foliar cover had a correlation coefficient of greater than 0.75. Some of the relationships, however, were site dependent
Effect of water-wall interaction potential on the properties of nanoconfined water
Much of the understanding of bulk liquids has progressed through study of the
limiting case in which molecules interact via purely repulsive forces, such as
a hard-core potential. In the same spirit, we report progress on the
understanding of confined water by examining the behavior of water-like
molecules interacting with planar walls via purely repulsive forces and compare
our results with those obtained for Lennard-Jones (LJ) interactions between the
molecules and the walls. Specifically, we perform molecular dynamics
simulations of 512 water-like molecules which are confined between two smooth
planar walls that are separated by 1.1 nm. At this separation, there are either
two or three molecular layers of water, depending on density. We study two
different forms of repulsive confinements, when the interaction potential
between water-wall is (i) and (ii) WCA-like repulsive potential. We
find that the thermodynamic, dynamic and structural properties of the liquid in
purely repulsive confinements qualitatively match those for a system with a
pure LJ attraction to the wall. In previous studies that include attractions,
freezing into monolayer or trilayer ice was seen for this wall separation.
Using the same separation as these previous studies, we find that the crystal
state is not stable with repulsive walls but is stable with WCA-like
repulsive confinement. However, by carefully adjusting the separation of the
plates with repulsive interactions so that the effective space
available to the molecules is the same as that for LJ confinement, we find that
the same crystal phases are stable. This result emphasizes the importance of
comparing systems only using the same effective confinement, which may differ
from the geometric separation of the confining surfaces.Comment: 20 pages, 10 figure
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