14,839 research outputs found
All Coronal Loops are the Same: Evidence to the Contrary
The 1998 April 20 spectral line data from the Coronal Diagnostics
Spectrometer (CDS) on the {\it Solar and Heliospheric Observatory} (\SOHO)
shows a coronal loop on the solar limb. Our original analysis of these data
showed that the plasma was multi-thermal, both along the length of the loop and
along the line of sight. However, more recent results by other authors indicate
that background subtraction might change these conclusions, so we consider the
effect of background subtraction on our analysis. We show Emission Measure (EM)
Loci plots of three representative pixels: loop apex, upper leg, and lower leg.
Comparisons of the original and background-subtracted intensities show that the
EM Loci are more tightly clustered after background subtraction, but that the
plasma is still not well represented by an isothermal model. Our results taken
together with those of other authors indicate that a variety of temperature
structures may be present within loops.Comment: Accepted for publication in ApJ Letter
The Reaction Process A+A->O in Sinai Disorder
The single-species reaction-diffusion process is examined in the
presence of an uncorrelated, quenched random velocity field. Utilising a
field-theoretic approach, we find that in two dimensions and below the density
decay is altered from the case of purely diffusing reactants. In two-dimensions
the density amplitude is reduced in the presence of weak disorder, yielding the
interesting result that Sinai disorder can cause reactions to occur at an {\it
increased} rate. This is in contrast to the case of long-range correlated
disorder, where it was shown that the reaction becomes sub-diffusion limited.
However, when written in terms of the microscopic diffusion constant it is seen
that increasing the disorder has the effect of reducing the rate of the
reaction. Below two dimensions, the effect of Sinai disorder is much more
severe and the reaction is shown to become sub-diffusion limited. Although
there is no universal amplitude for the time-dependence of the density, it is
universal when expressed in terms of the disorder-averaged diffusion length.
The appropriate amplitude is calculated to one-loop order.Comment: 12 pages, 2 figure
Universality in Turbulence: an Exactly Soluble Model
The present note contains the text of lectures discussing the problem of
universality in fully developed turbulence. After a brief description of
Kolmogorov's 1941 scaling theory of turbulence and a comparison between the
statistical approach to turbulence and field theory, we discuss a simple model
of turbulent advection which is exactly soluble but whose exact solution is
still difficult to analyze. The model exhibits a restricted universality. Its
correlation functions contain terms with universal but anomalous scaling but
with non-universal amplitudes typically diverging with the growing size of the
system. Strict universality applies only after such terms have been removed
leaving renormalized correlators with normal scaling. We expect that the
necessity of such an infrared renormalization is a characteristic feature of
universality in turbulence.Comment: 31 pages, late
On the exactly solvable pairing models for bosons
We propose the new exactly solvable model for bosons corresponding to the
attractive pairing interaction. Using the electrostatic analogy, the solution
of this model in thermodynamic limit is found. The transition from the
superfluid phase with the Bose condensate and the Bogoliubov - type spectrum of
excitations in the weak coupling regime to the incompressible phase with the
gap in the excitation spectrum in the strong coupling regime is observed.Comment: 19 page
Exact correlation functions of the BCS model in the canonical ensemble
We evaluate correlation functions of the BCS model for finite number of
particles. The integrability of the Hamiltonian relates it with the Gaudin
algebra . Therefore, a theorem that Sklyanin proved for the
Gaudin model, can be applied. Several diagonal and off-diagonal correlators are
calculated. The finite size scaling behavior of the pairing correlation
function is studied.Comment: 4 pages revtex; 2 figures .eps. Revised version to be published in
Phys. Rev. Let
A CDCL-style calculus for solving non-linear constraints
In this paper we propose a novel approach for checking satisfiability of
non-linear constraints over the reals, called ksmt. The procedure is based on
conflict resolution in CDCL style calculus, using a composition of symbolical
and numerical methods. To deal with the non-linear components in case of
conflicts we use numerically constructed restricted linearisations. This
approach covers a large number of computable non-linear real functions such as
polynomials, rational or trigonometrical functions and beyond. A prototypical
implementation has been evaluated on several non-linear SMT-LIB examples and
the results have been compared with state-of-the-art SMT solvers.Comment: 17 pages, 3 figures; accepted at FroCoS 2019; software available at
<http://informatik.uni-trier.de/~brausse/ksmt/
Blobs in Wolf-Rayet Winds: Random Photometric and Polarimetric Variability
Some isolated Wolf-Rayet stars present random variability in their optical
flux and polarization. We make the assumption that such variability is caused
by the presence of regions of enhanced density, i.e. blobs, in their envelopes.
In order to find the physical characteristics of such regions we have modeled
the stellar emission using a Monte Carlo code to treat the radiative transfer
in an inhomogeneous electron scattering envelope. We are able to treat multiple
scattering in the regions of enhanced density as well as in the envelope
itself. The finite sizes of the source and structures in the wind are also
taken into account. Most of the results presented here are based on a parameter
study of models with a single blob. The effects due to multiple blobs in the
envelope are considered to a more limited extent. Our simulations indicate that
the density enhancements must have a large geometric cross section in order to
produce the observed photopolarimetric variability. The sizes must be of the
order of one stellar radius and the blobs must be located near the base of the
envelope. These sizes are the same inferred from the widths of the sub-peaks in
optical emission lines of Wolf-Rayet stars. Other early-type stars show random
polarimetric fluctuations with characteristics similar to those observed in
Wolf-Rayet stars, which may also be interpreted in terms of a clumpy wind.
Although the origin of such structures is still unclear, the same mechanism may
be working in different types of hot stars envelopes to produce such
inhomogeneities.Comment: Accepted to ApJ. 17 pages + 6 figure
Transport and Boundary Scattering in Confined Geometries: Analytical Results
We utilize a geometric argument to determine the effects of boundary
scattering on the carrier mean-free path in samples of various cross sections.
Analytic expressions for samples with rectangular and circular cross sections
are obtained. We also outline a method for incorporating these results into
calculations of the thermal conductivity.Comment: 35 pages, Late
Leptonic constants of heavy quarkonia in potential approach of NRQCD
We consider a general scheme for calculating the leptonic constant of heavy
quarkonium QQ-bar in the framework of nonrelativistic quantum chromodynamics,
NRQCD, operating as the effective theory of nonrelativistic heavy quarks. We
explore the approach of static potential in QCD, which takes into account both
the evolution of effective charge in the three-loop approximation and the
linearly raising potential term, which provides the quark confinement. The
leptonic constants of bb-bar and cc-bar systems are evaluated by making use of
two-loop anomalous dimension for the current of nonrelativistic quarks, where
the factor for the normalization of matrix element is introduced in order to
preserve the renormalization group invariance of estimates.Comment: 18 pages, 6 eps-figures, discussion and references added, vNRQCD
analysis considere
Nuclear pairing: new perspectives
Nuclear pairing correlations are known to play an important role in various
single-particle and collective aspects of nuclear structure. After the first
idea by A. Bohr, B. Mottelson and D. Pines on similarity of nuclear pairing to
electron superconductivity, S.T. Belyaev gave a thorough analysis of the
manifestations of pairing in complex nuclei. The current revival of interest in
nuclear pairing is connected to the shift of modern nuclear physics towards
nuclei far from stability; many loosely bound nuclei are particle-stable only
due to the pairing. The theoretical methods borrowed from macroscopic
superconductivity turn out to be insufficient for finite systems as nuclei, in
particular for the cases of weak pairing and proximity of continuum states. We
suggest a simple numerical procedure of exact solution of the nuclear pairing
problem and discuss the physical features of this complete solution. We show
also how the continuum states can be naturally included in the consideration
bridging the gap between the structure and reactions. The path from coherent
pairing to chaos and thermalization and perspectives of new theoretical
approaches based on the full solution of pairing are discussed.Comment: 47 pages, 11 figure
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