527 research outputs found
Structure of Polyelectrolytes in Poor Solvent
We present simulations on charged polymers in poor solvent. First we
investigate in detail the dilute concentration range with and without imposed
extension constraints. The resulting necklace polymer conformations are
analyzed in detail. We find strong fluctuations in the number of pearls and
their sizes leading only to small signatures in the form factor and the
force-extension relation. The scaling of the peak in the structure factor with
the monomer density shows a pertinent different behavior from good solvent
chains.Comment: 7 pages, 5 figures. submitted to EP
Dynamics of Collapse of flexible Polyelectrolytes and Polyampholytes
We provide a theory for the dynamics of collapse of strongly charged
polyelectrolytes (PEs) and flexible polyampholytes (PAs) using Langevin
equation. After the initial stage, in which counterions condense onto PE, the
mechanism of approach to the globular state is similar for PE and PA. In both
instances, metastable pearl-necklace structures form in characteristic time
scale that is proportional to N^{4/5} where N is the number of monomers. The
late stage of collapse occurs by merger of clusters with the largest one
growing at the expense of smaller ones (Lifshitz- Slyozov mechanism). The time
scale for this process T_{COLL} N. Simulations are used to support the proposed
collapse mechanism for PA and PE.Comment: 14 pages, 2 figure
Persistence length of a polyelectrolyte in salty water: a Monte-Carlo study
We address the long standing problem of the dependence of the electrostatic
persistence length of a flexible polyelectrolyte (PE) on the screening
length of the solution within the linear Debye-Huckel theory. The
standard Odijk, Skolnick and Fixman (OSF) theory suggests ,
while some variational theories and computer simulations suggest . In this paper, we use Monte-Carlo simulations to study the conformation
of a simple polyelectrolyte. Using four times longer PEs than in previous
simulations and refined methods for the treatment of the simulation data, we
show that the results are consistent with the OSF dependence . The linear charge density of the PE which enters in the coefficient of
this dependence is properly renormalized to take into account local
fluctuations.Comment: 7 pages, 6 figures. Various corrections in text and reference
Type Ia Supernovae: Influence of the Initial Composition on the Nucleosynthesis, Light Curves, Spectra and Consequences for the Determination of Omega_M & Lambda
The influence of the initial composition of the exploding white dwarf on the
nucleosynthesis, light curves and spectra of Type Ia supernovae has been
studied in order to evaluate the size of evolutionary effects on cosmological
time scales, how the effects can be recognized and how one may be able to
correct for them.
The calculations are based on a set of delayed detonation models which give a
good account of the optical and infrared light curves and of the spectral
evolution. The explosions and light curves are calculated using a one-
dimensional Lagrangian radiation-hydro code including a nuclear network. NLTE-
spectra are computed for various epochs using the structure resulting from the
light curve code.
The following questions are addressed : What do we learn about the progenitor
evolution and its metallicity? What are the systematic effects for the
determination of the cosmological parameters and and how
can we recognize this potential 'pitfalls' and correct for evolutionary
effects?Comment: 19 pages, TeX, Ap
High spatial resolution X-ray spectroscopy of SNR Cassiopeia A with {\sl Chandra}
We present high spatial resolution X-ray spectroscopy of supernova remnant
Cassiopeia A with the {\sl Chandra} observations. The X-ray emitting region of
this remnant was divided into 38 34 pixels with a scale of 10\arcsec
10\arcsec each. Spectra of 960 pixels were created and fitted with
an absorbed two component non-equilibrium ionization model. With the spectral
analysis results we obtained maps of absorbing column density, temperatures,
ionization ages, and the abundances for Ne, Mg, Si, S, Ca and Fe. The Si, S and
possibly Ca abundance maps show obviously jet structures, while Fe doesn't
follow the jet but seems to be distributed perpendicular to it. In the range of
about two orders of magnitude, the abundances of Si, S and Ca show tight
correlations between each other, suggesting them to be ejecta from explosive
O-burning and incomplete Si-burning. Meanwhile, Ne abundance is well correlated
with that of Mg, indicating them to be the ashes of explosive C/Ne burning. The
Fe abundance is positively correlated with that of Si when Si abundance is
lower than 3 solar abundances, but a negative correlation appears when the Si
abundance is higher. We suggest that such a two phase correlation is the
results of different ways in which Fe is synthesized.Comment: accepted by ChJA
The Polarised Valence Quark Distribution from semi-inclusive DIS
The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of
opposite charge has been measured by the COMPASS experiment at CERN. The data
were collected in the years 2002-2004 using a 160 GeV polarised muon beam
scattered off a large polarised ^6LiD target and cover the range 0.006 < x <
0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry
A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an
evaluation of the first moment of Delta u_v + Delta d_v which is found to be
equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at
Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously
measured on the same data, this result favours a non-symmetric polarisation of
light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard
deviations, in contrast to the often assumed symmetric scenario Delta u-bar =
Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list
update
Spin alignment and violation of the OZI rule in exclusive and production in pp collisions
Exclusive production of the isoscalar vector mesons and is
measured with a 190 GeV proton beam impinging on a liquid hydrogen target.
Cross section ratios are determined in three intervals of the Feynman variable
of the fast proton. A significant violation of the OZI rule is found,
confirming earlier findings. Its kinematic dependence on and on the
invariant mass of the system formed by fast proton
and vector meson is discussed in terms of diffractive
production of resonances in competition with central
production. The measurement of the spin density matrix element of
the vector mesons in different selected reference frames provides another
handle to distinguish the contributions of these two major reaction types.
Again, dependences of the alignment on and on are
found. Most of the observations can be traced back to the existence of several
excited baryon states contributing to production which are absent in
the case of the meson. Removing the low-mass resonant
region, the OZI rule is found to be violated by a factor of eight,
independently of .Comment: 23 pages, 13 figures and 5 table
Thermonuclear Burning Regimes and the Use of SNe Ia in Cosmology
The calculations of the light curves of thermonuclear supernovae are carried
out by a method of multi-group radiation hydrodynamics. The effects of spectral
lines and expansion opacity are taken into account. The predictions for UBVI
fluxes are given. The values of rise time for B and V bands found in our
calculations are in good agreement with the observed values. We explain why our
results for the rise time have more solid physical justification than those
obtained by other authors. It is shown that small variations in the chemical
composition of the ejecta, produced in the explosions with different regimes of
nuclear burning, can influence drastically the light curve decline in the B
band and, to a lesser extent, in the V band. We argue that recent results on
positive cosmological constant Lambda, found from the high redshift supernova
observations, could be wrong in the case of possible variations of the
preferred mode of nuclear burning in the earlier Universe.Comment: 20 pages, 5 figures, presented at the conference "Astronomy at the
Eve of the New Century", Puschino, May 17-22, 1999. A few references and a
table added, typos correcte
The 3D Structure of N132D in the LMC: A Late-Stage Young Supernova Remnant
We have used the Wide Field Spectrograph (WiFeS) on the 2.3m telescope at
Siding Spring Observatory to map the [O III] 5007{\AA} dynamics of the young
oxygen-rich supernova remnant N132D in the Large Magellanic Cloud. From the
resultant data cube, we have been able to reconstruct the full 3D structure of
the system of [O III] filaments. The majority of the ejecta form a ring of
~12pc in diameter inclined at an angle of 25 degrees to the line of sight. We
conclude that SNR N132D is approaching the end of the reverse shock phase
before entering the fully thermalized Sedov phase of evolution. We speculate
that the ring of oxygen-rich material comes from ejecta in the equatorial plane
of a bipolar explosion, and that the overall shape of the SNR is strongly
influenced by the pre-supernova mass loss from the progenitor star. We find
tantalizing evidence of a polar jet associated with a very fast oxygen-rich
knot, and clear evidence that the central star has interacted with one or more
dense clouds in the surrounding ISM.Comment: Accepted for Publication in Astrophysics & Space Science, 18pp, 8
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