3,948 research outputs found
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Narrow absorption features in the co-added XMM-Newton RGS spectra of isolated Neutron Stars
We co-added the available XMM-Newton RGS spectra for each of the isolated
X-ray pulsars RX\,J0720.43125, RX\,J1308.6+2127 (RBS\,1223),
RX\,J1605.3+3249 and RX\,J1856.43754 (four members of the "Magnificent
Seven") and the "Three Musketeers" Geminga, PSR\,B0656+14 and PSR\,B1055-52. We
confirm the detection of a narrow absorption feature at 0.57 keV in the
co-added RGS spectra of RX\,J0720.43125 and RX\,J1605.3+3249 (including most
recent observations). In addition we found similar absorption features in the
spectra of RX\,J1308.6+2127 (at 0.53 keV) and maybe PSR\,B1055-52 (at 0.56
keV). The absorption feature in the spectra of RX\,J1308.6+2127 is broader than
the feature e.g. in RX\,J0720.43125. The narrow absorption features are
detected with 2 to 5.6 significance. Although very bright and
frequently observed, there are no absorption features visible in the spectra of
RX\,J1856.43754 and PSR\,B0656+14, while the co-added XMM-Newton RGS
spectrum of Geminga has not enough counts to detect such a feature. We discuss
a possible origin of these absorption features as lines caused by the presence
of highly ionised oxygen (in particular OVII and/or OVI at 0.57 keV) in the
interstellar medium and absorption in the neutron star atmosphere, namely the
absorption features at 0.57 keV as gravitational redshifted (=1.17)
OVIII.Comment: 14 pages, 10 figures and 10 tables. Accepted for publication by MNRAS
(Sep 12th, 2011
Control of Alternaria alternata, Causal Agent of Dead (Dormant) Flower Bud Disease of Pear
Dead (dormant) flower buds of pear are an important phenomenon in pear production in the Netherlands. Vigourous or unbalanced tree growth and Pseudomonas syringae pv. syringae (P.s.s.) are mentioned as likely causes of dead flower buds. Pseudomonas syringae pv. syringae was occasionally isolated from diseased flower buds. However, Alternaria alternata was nearly always isolated from diseased buds and also often in symptomless flower buds. By identifying the causal agent of dead flower buds disease, an effective control strategy can be developed. In field trials it was proven that fungicide treatments can reduce disease incidences significantly
The Extraordinarily Rapid Expansion of the X-ray Remnant of Kepler's Supernova (SN1604)
Four individual high resolution X-ray images from ROSAT and the Einstein
Observatory have been used to measure the expansion rate of the remnant of
Kepler's supernova (SN 1604). Highly significant measurements of the expansion
have been made for time baselines varying from 5.5 yrs to 17.5 yrs. All
measurements are consistent with a current expansion rate averaged over the
entire remnant of 0.239 (+/-0.015) (+0.017,-0.010) % per yr, which, when
combined with the known age of the remnant, determines the expansion parameter
m, defined as , to be 0.93 (+/-0.06) (+0.07,-0.04). The error
bars on these results include both statistical (first set of errors) and
systematic (second set) uncertainty. According to this result the X-ray remnant
is expanding at a rate that is remarkably close to free expansion and nearly
twice as fast as the mean expansion rate of the radio remnant. The expansion
rates as a function of radius and azimuthal angle are also presented based on
two ROSAT images that were registered to an accuracy better than 0.5
arcseconds. Significant radial and azimuthal variations that appear to arise
from the motion of individual X-ray knots are seen. The high expansion rate of
the X-ray remnant appears to be inconsistent with currently accepted dynamical
models for the evolution of Kepler's SNR.Comment: 14 pages, including 7 postscript figs, LaTeX, emulateapj. Accepted by
Ap
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
Phase separation in fluids exposed to spatially periodic external fields
We consider the liquid-vapor type phase transition for fluids confined within
spatially periodic external fields. For a fluid in d=3 dimensions, the periodic
field induces an additional phase, characterized by large density modulations
along the field direction. At the triple point, all three phases (modulated,
vapor, and liquid) coexist. At temperatures slightly above the triple point and
for low (high) values of the chemical potential, two-phase coexistence between
the modulated phase and the vapor (liquid) is observed. We study this
phenomenon using computer simulations and mean-field theory for the Ising
model. The theory shows that, in order for the modulated phase to arise, the
field wavelength must exceed a threshold value. We also find an extremely low
tension of the interface between the modulated phase and the vapor/liquid
phases. The tension is of the order 10^{-4} kB T per squared lattice spacing,
where kB is the Boltzmann constant, and T the temperature. In order to detect
such low tensions, a new simulation method is proposed. We also consider the
case of d=2 dimensions. The modulated phase then does not survive, leading to a
radically different phase diagram.Comment: 11 pages, 14 figure
1E 1547.0-5408: a radio-emitting magnetar with a rotation period of 2 seconds
The variable X-ray source 1E 1547.0-5408 was identified by Gelfand & Gaensler
(2007) as a likely magnetar in G327.24-0.13, an apparent supernova remnant. No
X-ray pulsations have been detected from it. Using the Parkes radio telescope,
we discovered pulsations with period P = 2.069 s. Using the Australia Telescope
Compact Array, we localized these to 1E 1547.0-5408. We measure dP/dt =
(2.318+-0.005)e-11, which for a magnetic dipole rotating in vacuo gives a
surface field strength of 2.2e14 G, a characteristic age of 1.4 kyr, and a
spin-down luminosity of 1.0e35 ergs/s. Together with its X-ray characteristics,
these rotational parameters of 1E 1547.0-5408 prove that it is a magnetar, only
the second known to emit radio waves. The distance is ~9 kpc, derived from the
dispersion measure of 830 pc/cc. The pulse profile at a frequency of 1.4 GHz is
extremely broad and asymmetric due to multipath propagation in the ISM, as a
result of which only approximately 75% of the total flux at 1.4 GHz is pulsed.
At higher frequencies the profile is more symmetric and has FWHM = 0.12P.
Unlike in normal radio pulsars, but in common with the other known
radio-emitting magnetar, XTE J1810-197, the spectrum over 1.4-6.6 GHz is flat
or rising, and we observe large, sudden changes in the pulse shape. In a
contemporaneous Swift X-ray observation, 1E 1547.0-5408 was detected with
record high flux, f_X(1-8 keV) ~ 5e-12 ergs/cm^2/s, 16 times the historic
minimum. The pulsar was undetected in archival radio observations from 1998,
implying a flux < 0.2 times the present level. Together with the transient
behavior of XTE J1810-197, these results suggest that radio emission is
triggered by X-ray outbursts of usually quiescent magnetars.Comment: Accepted for publication in ApJ Letter
Evidence for precession of the isolated neutron star RX J0720.4-3125
The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 obtained
over 4.5 years can be described by sinusoidal variations in the inferred
blackbody temperature, the size of the emitting area and the depth of the
absorption line with a period of 7.1 +/- 0.5 years, which we suggest to be the
precession period of the neutron star. Precession of a neutron star with two
hot spots of different temperature and size, probably not located exactly in
antipodal positions, may account for the variations in the X-ray spectra,
changes in the pulsed fraction, shape of the light curve and the phase-lag
between soft and hard energy bands observed from RX J0720.4-3125. An
independent sinusoidal fit to published and new pulse timing residuals from a
coherent analysis covering ~12 years yields a consistent period of 7.7 +/- 0.6
years supporting the precession model.Comment: Accepted for publication in A&A Letters, 5 pages, 5 figure
Critical behavior in colloid-polymer mixtures: theory and simulation
We extensively investigated the critical behavior of mixtures of colloids and
polymers via the two-component Asakura-Oosawa model and its reduction to a
one-component colloidal fluid using accurate theoretical and simulation
techniques. In particular the theoretical approach, hierarchical reference
theory [Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of
long-range fluctuations on phase separation giving exponents which differ
strongly from their mean-field values, and are in good agreement with those of
the three-dimensional Ising model. Computer simulations combined with
finite-size scaling analysis confirm the Ising universality and the accuracy of
the theory, although some discrepancy in the location of the critical point
between one-component and full-mixture description remains. To assess the limit
of the pair-interaction description, we compare one-component and two-component
results.Comment: 15 pages, 10 figures. Submitted to Phys. Rev.
SurF: an innovative framework in biosecurity and animal health surveillance evaluation
Surveillance for biosecurity hazards is being conducted by the New Zealand Competent Authority, the Ministry for Primary Industries (MPI) to support New Zealand's biosecurity system. Surveillance evaluation should be an integral part of the surveillance life cycle, as it provides a means to identify and correct problems and to sustain and enhance the existing strengths of a surveillance system. The surveillance evaluation Framework (SurF) presented here was developed to provide a generic framework within which the MPI biosecurity surveillance portfolio, and all of its components, can be consistently assessed. SurF is an innovative, crossâsectoral effort that aims to provide a common umbrella for surveillance evaluation in the animal, plant, environment and aquatic sectors. It supports the conduct of the following four distinct components of an evaluation project: (i) motivation for the evaluation, (ii) scope of the evaluation, (iii) evaluation design and implementation and (iv) reporting and communication of evaluation outputs. Case studies, prepared by MPI subject matter experts, are included in the framework to guide users in their assessment. Three case studies were used in the development of SurF in order to assure practical utility and to confirm usability of SurF across all included sectors. It is anticipated that the structured approach and information provided by SurF will not only be of benefit to MPI but also to other New Zealand stakeholders. Although SurF was developed for internal use by MPI, it could be applied to any surveillance system in New Zealand or elsewhere
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