871 research outputs found
HD 24355 observed by the Kepler K2 mission: a rapidly oscillating Ap star pulsating in a distorted quadrupole mode
We present an analysis of the first Kepler K2 mission observations of a rapidly oscillating Ap (roAp) star, HD 24355 (V = 9.65). The star was discovered in SuperWASP broad-band photometry with a frequency of 224.31 dâ1 (2596.18 ÎŒHz; P = 6.4 min) and an amplitude of
1.51 mmag, with later spectroscopic analysis of low-resolution spectra showing HD 24355 to
be an A5 Vp SrEu star. The high-precision K2 data allow us to identify 13 rotationally split sidelobes to the main pulsation frequency of HD 24355. This number of sidelobes combined with an unusual rotational phase variation show this star to be the most distorted quadrupole roAp pulsator yet observed. In modelling this star, we are able to reproduce well the amplitude modulation of the pulsation, and find a close match to the unusual phase variations. We show this star to have a pulsation frequency higher than the critical cut-off frequency. This is currently the only roAp star observed with the Kepler spacecraft in short cadence mode that has a photometric amplitude detectable from the ground, thus allowing comparison between the mmag amplitude ground-based targets and the ÎŒmag spaced-based discoveries. No further pulsation modes are identified in the K2 data, showing this star to be a single-mode pulsator
Analysis of a 290-Year Net Accumulation Time Series from Mt. Logan, Yukon
A 102.5-m mechanically continuous firn and ice core sequence retrieved from the Northwest Col of Mt. Logan (latitude 60°30\u27N; longitude 140°35\u27W; site location 5340 m a.s.l.) in the Yukon Territory, Canada, has been analyzed continuously for stable isotopes, pH and liquid electrolytic conductivity. Specific sections of the core have been analyzed for total ÎČ-activity (0-22 m) and trace ion concentrations (across major volcanic events) in order to date the core. In the lower half of the core, nitrate and some other ionic species are used to identify annual increments except between AD 1693 and AD 1720 and between AD 1729 and AD 1735 where only average annual increments are given. Annual increments were converted to water equivalents, then corrected for ice flow thinning as well as for origin, since a significant net accumulation gradient exists across the borehole site. The time series was subjected to cross correlation analysis, using instrumental data for the last 80 years, and to spectral analyses, using a 250-year sequence
Universal Fluctuations of the Danube Water Level: a Link with Turbulence, Criticality and Company Growth
A global quantity, regardless of its precise nature, will often fluctuate
according to a Gaussian limit distribution. However, in highly correlated
systems, other limit distributions are possible. We have previously calculated
one such distribution and have argued that this function should apply
specifically, and in many instances, to global quantities that define a steady
state. Here we demonstrate, for the first time, the relevance of this
prediction to natural phenomena. The river level fluctuations of the Danube are
observed to obey our prediction, which immediately establishes a generic
statistical connection between turbulence, criticality and company growth
statistics.Comment: 5 pages, 1 figur
Cues and knowledge structures used by mental-health professionals when making risk assessments
Background: Research into mental-health risks has tended to focus on epidemiological approaches and to consider pieces of evidence in isolation. Less is known about the particular
factors and their patterns of occurrence that influence cliniciansâ risk judgements in practice.
Aims: To identify the cues used by clinicians to make risk judgements and to explore how these combine within cliniciansâ psychological representations of suicide, self-harm, self-neglect, and harm to others.
Method: Content analysis was applied to semi-structured interviews conducted with 46 practitioners from various mental-health disciplines, using mind maps to represent the
hierarchical relationships of data and concepts.
Results: Strong consensus between experts meant their knowledge could be integrated into a single hierarchical structure for each risk. This revealed contrasting emphases between data and concepts underpinning risks, including: reflection and forethought for suicide; motivation
for self-harm; situation and context for harm to others; and current presentation for self-neglect.
Conclusions: Analysis of expertsâ risk-assessment knowledge identified influential cues and their relationships to risks. It can inform development of valid risk-screening decision support systems that combine actuarial evidence with clinical expertise
Ferroelectric and Dipolar Glass Phases of Non-Crystalline Systems
In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly
discussed the existence and nature of ferroelectric order in positionally
disordered dipolar materials. Here we report further results and give a
complete description of our work. Simulations of randomly frozen and
dynamically disordered dipolar soft spheres are used to study ferroelectric
ordering in non-crystalline systems. We also give a physical interpretation of
the simulation results in terms of short- and long-range interactions. Cases
where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models,
respectively) are considered. It is found that the Ising model displays
ferroelectric phases in frozen amorphous systems, while the XY and XYZ models
form dipolar glass phases at low temperatures. In the dynamically disordered
model the equations of motion are decoupled such that particle translation is
completely independent of the dipolar forces. These systems spontaneously
develop long-range ferroelectric order at nonzero temperature despite the
absence of any fined-tuned short-range spatial correlations favoring dipolar
order. Furthermore, since this is a nonequilibrium model we find that the
paraelectric to ferroelectric transition depends on the particle mass. For the
XY and XYZ models, the critical temperatures extrapolate to zero as the mass of
the particle becomes infinite, whereas, for the Ising model the critical
temperature is almost independent of mass and coincides with the ferroelectric
transition found for the randomly frozen system at the same density. Thus in
the infinite mass limit the results of the frozen amorphous systems are
recovered.Comment: 25 pages (LATEX, no macros). 11 POSTSCRIPT figures enclosed.
Submitted to Phisical Review E. Contact: [email protected]
Universal Fluctuations in Correlated Systems
The probability density function (PDF) of a global measure in a large class
of highly correlated systems has been suggested to be of the same functional
form. Here, we identify the analytical form of the PDF of one such measure, the
order parameter in the low temperature phase of the 2D-XY model. We demonstrate
that this function describes the fluctuations of global quantities in other
correlated, equilibrium and non-equilibrium systems. These include a coupled
rotor model, Ising and percolation models, models of forest fires, sand-piles,
avalanches and granular media in a self organized critical state. We discuss
the relationship with both Gaussian and extremal statistics.Comment: 4 pages, 2 figure
Tidal controls on the flow of ice streams
The flow of many Antarctic ice streams is known to be significantly influenced by tides. In the past, modeling studies have implemented the tidal forces acting on a coupled ice stream/ice shelf system in a number of different ways, but the consequences that this has on the modeled response of ice streams to tides have, until now, not been considered. Here we investigate for the first time differences in model response that are only due to differences in the way tidal forcings are implemented. We find that attempts to simplify the problem by neglecting flexural stresses are generally not valid and forcing models with only changes in ocean back pressure will not capture either the correct amplitudes or length scale
Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181
We report the frequency analysis of a known roAp star, HD 86181 (TIC 469246567), with new inferences from TESS data. We derive the rotation frequency to be Îœrot = 0.48753 ± 0.00001dâ1. The pulsation frequency spectrum is rich, consisting of two doublets and one quintuplet, which we interpret to be oblique pulsation multiplets from consecutive, high-overtone dipole, quadrupole and dipole modes. The central frequency of the quintuplet is 232.7701dâ1 (2.694 mHz). The phases of the sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show that the quadrupole mode is distorted. Following the oblique pulsator model, we calculate the rotation inclination, i, and magnetic obliquity, ÎČ, of this star, which provide detailed information about the pulsation geometry. The i and ÎČ derived from the best fit of the pulsation amplitude and phase modulation to a theoretical model, including the magnetic field effect, slightly differ from those calculated for a pure quadrupole, indicating the contributions from l = 4, 6, 8, ... are small. Non-adiabatic models with different envelope convection conditions and physics configurations were considered for this star. It is shown that models with envelope convection almost fully suppressed can explain the excitation at the observed pulsation frequencies
Rheological constitutive equation for model of soft glassy materials
We solve exactly and describe in detail a simplified scalar model for the low
frequency shear rheology of foams, emulsions, slurries, etc. [P. Sollich, F.
Lequeux, P. Hebraud, M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)]. The model
attributes similarities in the rheology of such ``soft glassy materials'' to
the shared features of structural disorder and metastability. By focusing on
the dynamics of mesoscopic elements, it retains a generic character.
Interactions are represented by a mean-field noise temperature x, with a glass
transition occurring at x=1 (in appropriate units). The exact solution of the
model takes the form of a constitutive equation relating stress to strain
history, from which all rheological properties can be derived. For the linear
response, we find that both the storage modulus G' and the loss modulus G''
vary with frequency as \omega^{x-1} for 1<x<2, becoming flat near the glass
transition. In the glass phase, aging of the moduli is predicted. The steady
shear flow curves show power law fluid behavior for x<2, with a nonzero yield
stress in the glass phase; the Cox-Merz rule does not hold in this
non-Newtonian regime. Single and double step strains further probe the
nonlinear behavior of the model, which is not well represented by the BKZ
relation. Finally, we consider measurements of G' and G'' at finite strain
amplitude \gamma. Near the glass transition, G'' exhibits a maximum as \gamma
is increased in a strain sweep. Its value can be strongly overestimated due to
nonlinear effects, which can be present even when the stress response is very
nearly harmonic. The largest strain \gamma_c at which measurements still probe
the linear response is predicted to be roughly frequency-independent.Comment: 24 pages, REVTeX, uses multicol, epsf and amssymp; 20 postscript
figures (included). Minor changes to text (relation to mode coupling theory,
update on recent foam simulations etc.) and figures (emphasis on low
frequency regime); typos corrected and reference added. Version to appear in
Physical Review
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