2,513 research outputs found
Revisiting the theoretical DBV (V777 Her) instability strip: the MLT theory of convection
We reexamine the theoretical instability domain of pulsating DB white dwarfs
(DBV or V777 Her variables). We performed an extensive -mode nonadiabatic
pulsation analysis of DB evolutionary models considering a wide range of
stellar masses, for which the complete evolutionary stages of their progenitors
from the ZAMS, through the thermally pulsing AGB and born-again phases, the
domain of the PG1159 stars, the hot phase of DO white dwarfs, and then the DB
white dwarf stage have been considered. We explicitly account for the evolution
of the chemical abundance distribution due to time-dependent chemical diffusion
processes. We examine the impact of the different prescriptions of the MLT
theory of convection and the effects of small amounts of H in the almost
He-pure atmospheres of DB stars on the precise location of the theoretical blue
edge of the DBV instability strip.Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 200
Hydrodynamic theory of an electron gas
The generalised hydrodynamic theory of an electron gas, which does not rely
on an assumption of a local equilibrium, is derived as the long-wave limit of a
kinetic equation. Apart from the common hydrodynamics variables the theory
includes the tensor fields of the higher moments of the distribution function.
In contrast to the Bloch hydrodynamics, the theory leads to the correct plasmon
dispersion and in the low frequency limit recovers the Navies-Stocks
hydrodynamics. The linear approximation to the generalised hydrodynamics is
closely related to the theory of highly viscous fluids.Comment: 4 pages, revte
Microscopic mechanism for mechanical polishing of diamond (110) surfaces
Mechanically induced degradation of diamond, as occurs during polishing, is
studied using total--energy pseudopotential calculations. The strong asymmetry
in the rate of polishing between different directions on the diamond (110)
surface is explained in terms of an atomistic mechanism for nano--groove
formation. The post--polishing surface morphology and the nature of the
polishing residue predicted by this mechanism are consistent with experimental
evidence.Comment: 4 pages, 5 figure
The stigmatisation of people with chronic back pain
This study responded to the need for better theoretical understanding of experiences that shape the beliefs, attitudes and needs of chronic back patients attending pain clinics. The aim was explore and conceptualise the experiences of people of working age who seek help from pain clinics for chronic back pain. Methods. This was a qualitative study, based on an interpretative phenomenological approach (IPA). During in-depth interviews in their homes, participants were invited to 'tell their story' from the time their pain began. Participants were twelve male and six female patients, aged between 28 and 62 years, diagnosed as having chronic benign back pain. All had recently attended one of two pain clinics as new referrals. The interview transcripts were analysed thematically. Findings. Stigmatisation emerged as a key theme from the narrative accounts of participants. The findings expose subtle as well as overt stigmatising responses by family, friends, health professionals and the general public which appeared to have a profound effect on the perceptions, self esteem and behaviours of those interviewed. Conclusions. The findings suggest that patients with chronic back pain feel stigmatised by the time they attend pain clinics and this may affect their attitudes and behaviours towards those offering professional help. Theories of chronic pain need to accommodate these responses, while pain management programmes need to address the realities and practicalities of dealing with stigma in everyday life
Basis Functions for Linear-Scaling First-Principles Calculations
In the framework of a recently reported linear-scaling method for
density-functional-pseudopotential calculations, we investigate the use of
localized basis functions for such work. We propose a basis set in which each
local orbital is represented in terms of an array of `blip functions'' on the
points of a grid. We analyze the relation between blip-function basis sets and
the plane-wave basis used in standard pseudopotential methods, derive criteria
for the approximate equivalence of the two, and describe practical tests of
these criteria. Techniques are presented for using blip-function basis sets in
linear-scaling calculations, and numerical tests of these techniques are
reported for Si crystal using both local and non-local pseudopotentials. We
find rapid convergence of the total energy to the values given by standard
plane-wave calculations as the radius of the linear-scaling localized orbitals
is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty,
submitted to Phys. Rev.
Timesaving Double-Grid Method for Real-Space Electronic-Structure Calculations
We present a simple and efficient technique in ab initio electronic-structure
calculation utilizing real-space double-grid with a high density of grid points
in the vicinity of nuclei. This technique promises to greatly reduce the
overhead for performing the integrals that involves non-local parts of
pseudopotentials, with keeping a high degree of accuracy. Our procedure gives
rise to no Pulay forces, unlike other real-space methods using adaptive
coordinates. Moreover, we demonstrate the potential power of the method by
calculating several properties of atoms and molecules.Comment: 4 pages, 5 figure
Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals
A theoretical study of the magnon-drag Peltier and Seebeck effects in
ferromagnetic metals is presented. A magnon heat current is described
perturbatively from the microscopic viewpoint with respect to electron--magnon
interactions and the electric field. Then, the magnon-drag Peltier coefficient
\Pi_\MAG is obtained as the ratio between the magnon heat current and the
electric charge current. We show that \Pi_\MAG=C_\MAG T^{5/2} at a low
temperature ; that the coefficient C_\MAG is proportional to the spin
polarization of the electric conductivity; and that for C_\MAG<0,
but . From experimental results for magnon-drag Peltier
effects, we estimate that the strength of the electron--magnon interaction is
about 0.3 eV for permalloy.Comment: 3 pages, 2 figures, accepted for publication in Journal of the
Physical Society of Japa
Entangled-photon Fourier optics
Entangled photons, generated by spontaneous parametric down-conversion from a
second-order nonlinear crystal, present a rich potential for imaging and
image-processing applications. Since this source is an example of a three-wave
mixing process, there is more flexibility in the choices of illumination and
detection wavelengths and in the placement of object(s) to be imaged. Moreover,
this source is entangled, a fact that allows for imaging configurations and
capabilities that cannot be achieved using classical sources of light. In this
paper we examine a number of imaging and image-processing configurations that
can be realized using this source. The formalism that we utilize facilitates
the determination of the dependence of imaging resolution on the physical
parameters of the optical arrangement.Comment: 41 pages, 12 figures, accepted for publication in J. Opt. Soc. Am.
Towards a pure ZZ Ceti instability strip
We have observed again two stars inside the ZZ Ceti instability strip that
were previously classified as not-observed-to-vary (NOV) by Mukadam et al.
(2004) and found them to be low-amplitude variables. Some evidence points to a
pure ZZ Ceti instability strip; other evidence contests it. The two stars
previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic
effective temperatures that place them inside the ZZ Ceti instability strip,
and they were "contaminating" the strip as constant stars, which could indicate
that the instability strip was no longer a simple evolutionary stage. A pure
instability strip indicates that pulsation is a normal phase which all DAs must
go through. We used effective temperatures derived from SDSS optical spectra by
comparing them with model atmospheres to look for pulsators through
time-resolved photometry and stars previously classified as NOV. Our new
results indicate, but do not prove, a pure instability strip, because there are
still other NOV stars that need to be observed again. Additionally, we have
discovered five other ZZ Ceti stars based on their effective temperatures.Comment: 12 pages, 3 figur
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