991 research outputs found
Three-body treatment of the penetration through the Coulomb field of a two-fragment nucleus
On the basis of the Faddeev integral equations method and the Watson-
Feshbach concept of the effective (optical) interaction potential, the first
fully consistent three-body approach to the description of the penetration of a
charged particle through the Coulomb field of a two-particle bound complex
(composed of one charged and one neutral particles) has been developed. A
general formalism has been elaborated and on its basis, to a first
approximation in the Sommerfeld parameter, the influence of the nuclear
structure on the probability of the penetration of a charged particle (the
muon, the pion, the kaon and the proton) through the Gamow barrier of a
two-fragment nucleus (the deuteron and the two lightest lambda hypernuclei,
lambda hypertriton and lambda hyperhelium-5, has been calculated and studied.Comment: LaTeX, 30 pages, 4 eps figure
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Selecting Mathematical Software for Dependability Assessment of Computer Systems Described by Stiff Markov Chains
Markov and semi-Markov models are widely used in dependability assessment of complex computer-based systems. Model stiffness poses a serious problem both in terms of computational difficulties and in terms of accuracy of the assessment. Selecting an appropriate method and software package for solving stiff Markov models proved to be a non-trivial task. In this paper we provide an empirical comparison of two approaches to dealing with stiffness â stiffness avoidance and stiffness-tolerance. The study includes several well known techniques and software tools used for solving Kolmogorovâs differential equations derived from complex stiff Markov models. In the comparison we used realistic cases studies developed by others in the past: i) a computer system with hardware redundancy and diverse software, and ii) a queuing system with a server break-down and repair. The results indicate that the accuracy of the known methods is significantly affected by the stiffness of the Markov models, which led us to developing a procedure (an algorithm) for selecting the optimal method and tool for solving a given stiff Markov model. The algorithm is, also included in the paper
Shape parameters of Galactic open clusters
(abridged) In this paper we derive observed and modelled shape parameters
(apparent ellipticity and orientation of the ellipse) of 650 Galactic open
clusters identified in the ASCC-2.5 catalogue. We provide the observed shape
parameters of Galactic open clusters, computed with the help of a
multi-component analysis. For the vast majority of clusters these parameters
are determined for the first time. High resolution ("star by star") N-body
simulations are carried out with the specially developed GRAPE code
providing models of clusters of different initial masses, Galactocentric
distances and rotation velocities. The comparison of models and observations of
about 150 clusters reveals ellipticities of observed clusters which are too low
(0.2 vs. 0.3), and offers the basis to find the main reason for this
discrepancy. The models predict that after Myr clusters reach an
oblate shape with an axes ratio of , and with the major axis
tilted by an angle of with respect to the
Galactocentric radius due to differential rotation of the Galaxy. Unbiased
estimates of cluster shape parameters require reliable membership determination
in large cluster areas up to 2-3 tidal radii where the density of cluster stars
is considerably lower than the background. Although dynamically bound stars
outside the tidal radius contribute insignificantly to the cluster mass, their
distribution is essential for a correct determination of cluster shape
parameters. In contrast, a restricted mass range of cluster stars does not play
such a dramatic role, though deep surveys allow to identify more cluster
members and, therefore, to increase the accuracy of the observed shape
parameters.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
A Search for EUV Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7
(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion
passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).
We obtained contemporaneous data on Comet NEAT Q4 with the X-ray
Observatory ACIS instrument, marking the first simultaneous EUV and X-ray
spectral observations of a comet. The total CHIPS/EUV observing times were 337
ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and
we calculate we have captured all the comet flux in the instrument
field of view. We set upper limits on solar wind charge exchange emission lines
of O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrum
of Q4 obtained with can be reproduced by modeling emission lines of
C, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission line
intensities are consistent with our predictions from a solar wind charge
exchange model. The model predictions for the EUV emission line intensities are
determined from the intensity ratios of the cascading X-ray and EUV photons
arising in the charge exchange processes. They are compatible with the measured
limits on the intensities of the EUV lines. For comet Q4, we measured a total
X-ray flux of 3.7 ergs cm s, and derive from
model predictions a total EUV flux of 1.5 erg cm
s. The CHIPS observations occurred predominantly while the satellite was
on the dayside of Earth. For much of the observing time, CHIPS performed
observations at smaller solar angles than it was designed for and EUV emission
from the Sun scattered into the instrument limited the sensitivity of the EUV
measurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical
Journa
Long-range behavior of the optical potential for the elastic scattering of charged composite particles
The asymptotic behavior of the optical potential, describing elastic
scattering of a charged particle off a bound state of two charged, or
one charged and one neutral, particles at small momentum transfer
or equivalently at large intercluster distance
, is investigated within the framework of the exact three-body
theory. For the three-charged-particle Green function that occurs in the exact
expression for the optical potential, a recently derived expression, which is
appropriate for the asymptotic region under consideration, is used. We find
that for arbitrary values of the energy parameter the non-static part of the
optical potential behaves for as
. From this we derive for the
Fourier transform of its on-shell restriction for the behavior , i.e.,
dipole or quadrupole terms do not occur in the coordinate-space asymptotics.
This result corroborates the standard one, which is obtained by perturbative
methods. The general, energy-dependent expression for the dynamic
polarisability is derived; on the energy shell it reduces to the
conventional polarisability which is independent of the energy. We
emphasize that the present derivation is {\em non-perturbative}, i.e., it does
not make use of adiabatic or similar approximations, and is valid for energies
{\em below as well as above the three-body dissociation threshold}.Comment: 35 pages, no figures, revte
Galactic Rotation Parameters from Data on Open Star Clusters
Currently available data on the field of velocities Vr, Vl, Vb for open star
clusters are used to perform a kinematic analysis of various samples that
differ by heliocentric distance, age, and membership in individual structures
(the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters
located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the
Galactic rotation parameters
Wo =-26.0+-0.3 km/s/kpc,
W'o = 4.18+-0.17 km/s/kpc^2,
W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1
km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo =
0+-1 degrees. The Galactocentric distance Ro in the model used has been found
to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7
kpc and 5.6+-0.3 kpc for the samples of young (50 Myr)
clusters, respectively. Our study of the kinematics of young open star clusters
in various spiral arms has shown that the kinematic parameters are similar to
the parameters obtained from the entire sample for the Carina-Sagittarius and
Perseus arms and differ significantly from them for the Orion arm. The
contraction effect is shown to be typical of star clusters with various ages.
It is most pronounced for clusters with a mean age of 100 Myr, with the
contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table
Properties of pattern formation and selection processes in nonequilibrium systems with external fluctuations
We extend the phase field crystal method for nonequilibrium patterning to
stochastic systems with external source where transient dynamics is essential.
It was shown that at short time scales the system manifests pattern selection
processes. These processes are studied by means of the structure function
dynamics analysis. Nonequilibrium pattern-forming transitions are analyzed by
means of numerical simulations.Comment: 15 poages, 8 figure
Renormalization of the Three-Body System with Short-Range Interactions
We discuss renormalization of the non-relativistic three-body problem with
short-range forces. The problem becomes non-perturbative at momenta of the
order of the inverse of the two-body scattering length, and an infinite number
of graphs must be summed. This summation leads to a cutoff dependence that does
not appear in any order in perturbation theory. We argue that this cutoff
dependence can be absorbed in a single three-body counterterm and compute the
running of the three-body force with the cutoff. We comment on relevance of
this result for the effective field theory program in nuclear and molecular
physics.Comment: 5 pages, RevTex, 4 PS figures included with epsf.sty, some clarifying
comments added, version to appear in Phys. Rev. Let
Expression dynamics of a cellular metabolic network
Toward the goal of understanding system properties of biological networks, we investigate the global and local regulation of gene expression in the Saccharomyces cerevisiae metabolic network. Our results demonstrate predominance of local gene regulation in metabolism. Metabolic genes display significant coexpression on distances smaller than the average network distance, a behavior supported by the distribution of transcription factor binding sites in the metabolic network and genome context associations. Positive gene coexpression decreases monotonically with distance in the network, while negative coexpression is strongest at intermediate network distances. We show that basic topological motifs of the metabolic network exhibit statistically significant differences in coexpression behavior
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