7,359 research outputs found
Regional-scale risk assessment methodology using the Relative Risk Model (RRM) for surface freshwater aquatic ecosystems in South Africa
To maximise the long-term use of limited ecosystem services in South Africa, managers continually require approaches to optimise the establishment of balances between the use and protection of ecosystems to ensure sustainability. Surface freshwater aquatic ecosystems are dynamic and difficult to manage effectively. Sound management protocols that can identify and rank threats to these ecosystems are urgently required. The Regional-Scale Risk Assessment approach is carried out ona spatial scale and allows for the consideration of multiple sources of multiple stressors affecting multiple endpoints, with the inclusion of local ecosystem dynamics and the characteristics of the landscape that may affect the risk estimate. This paper presents an integrated approach to carry out regional-scale ecological risk assessments using a Relative Risk Model (RRM) adapted for South African conditions. The RRM consists of 10 procedural steps that are relatively easily applied. The use and application of the RRM within South Africa has the potential to provide resource users, resource conservators and regulators of surface aquatic ecosystems with a range of benefits. These benefits include the establishment of a validated,structured methodology that is sensitive to the dynamics of individual case studies, extremely informative, locally applicable and internationally comparable with other RRM assessments. The use of the RRM approach in South Africa has many advantages that outweigh some disadvantages. This approach has the potential to substantially contribute towards the ease and effectiveness of management of the balance between the use and protection of aquatic ecosystems in South Africa
Evidence for nonlinear diffusive shock acceleration of cosmic-rays in the 2006 outburst of the recurrent nova RS Ophiuchi
Spectroscopic observations of the 2006 outburst of the recurrent nova RS
Ophiuchi at both infrared (IR) and X-ray wavelengths have shown that the blast
wave has decelerated at a higher rate than predicted by the standard
test-particle adiabatic shock-wave model. Here we show that the observed
evolution of the nova remnant can be explained by the diffusive shock
acceleration of particles at the blast wave and the subsequent escape of the
highest energy ions from the shock region. Nonlinear particle acceleration can
also account for the difference of shock velocities deduced from the IR and
X-ray data. The maximum energy that accelerated electrons and protons can have
achieved in few days after outburst is found to be as high as a few TeV. Using
the semi-analytic model of nonlinear diffusive shock acceleration developed by
Berezhko & Ellison, we show that the postshock temperature of the shocked gas
measured with RXTE/PCA and Swift/XRT imply a relatively moderate acceleration
efficiency.Comment: Accepted for publication in ApJ
The Extent and Cause of the Pre-White Dwarf Instability Strip
One of the least understood aspects of white dwarf evolution is the process
by which they are formed. We are aided, however, by the fact that many H- and
He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators.
Pulsations in PWDs provide a unique opportunity to probe their interiors, which
are otherwise inaccesible to direct observation. Until now, however, the nature
of the pulsation mechanism, the precise boundaries of the instability strip,
and the mass distribution of the PWDs were complete mysteries. These problems
must be addressed before we can apply knowledge of pulsating PWDs to improve
understanding of white dwarf formation. This paper lays the groundwork for
future theoretical investigations of these stars. In recent years, Whole Earth
Telescope observations led to determination of mass and luminosity for the
majority of the (non-central star) PWD pulsators. With these observations, we
identify the common properties and trends PWDs exhibit as a class. We find that
pulsators of low mass have higher luminosity, suggesting the range of
instability is highly mass-dependent. The observed trend of decreasing periods
with decreasing luminosity matches a decrease in the maximum (standing-wave)
g-mode period across the instability strip. We show that the red edge can be
caused by the lengthening of the driving timescale beyond the maximum
sustainable period. This result is general for ionization-based driving
mechanisms, and it explains the mass-dependence of the red edge. The observed
form of the mass-dependence provides a vital starting point for future
theoretical investigations of the driving mechanism. We also show that the blue
edge probably remains undetected because of selection effects arising from
rapid evolution.Comment: 40 pages, 6 figures, accepted by ApJ Oct 27, 199
Near-field interaction between domain walls in adjacent Permalloy nanowires
The magnetostatic interaction between two oppositely charged transverse
domain walls (DWs)in adjacent Permalloy nanowires is experimentally
demonstrated. The dependence of the pinning strength on wire separation is
investigated for distances between 13 and 125 nm, and depinning fields up to 93
Oe are measured. The results can be described fully by considering the
interaction between the full complex distribution of magnetic charge within
rigid, isolated DWs. This suggests the DW internal structure is not appreciably
disturbed by the pinning potential, and that they remain rigid although the
pinning strength is significant. This work demonstrates the possibility of
non-contact DW trapping without DW perturbation and full continuous flexibility
of the pinning potential type and strength. The consequence of the interaction
on DW based data storage schemes is evaluated.Comment: 4 pages, 4 figures, 1 page supplimentary material (supporting.ps
Infrared afterglow of GRB041219 as a result of reradiation on dust in a circumstellar cloud
Observations of gamma ray bursts (GRB) afterglows in different spectral bands
provide a most valuable information about their nature, as well as about
properties of surrounding medium. Powerful infrared afterglow was observed from
the strong GRB041219. Here we explain the observed IR afterglow in the model of
a dust reradiation of the main GRB signal in the envelope surrounding the GRB
source. In this model we do not expect appearance of the prompt optical
emission which should be absorbed in the dust envelope. We estimate the
collimation angle of the gamma ray emission, and obtain restrictions on the
redshift (distance to GRB source), by fitting the model parameters to the
observational data.Comment: 6 pages, 2 figures, Submited to Astrofizik
From Euclidean to Minkowski space with the Cauchy-Riemann equations
We present an elementary method to obtain Green's functions in
non-perturbative quantum field theory in Minkowski space from calculated
Green's functions in Euclidean space. Since in non-perturbative field theory
the analytical structure of amplitudes is many times unknown, especially in the
presence of confined fields, dispersive representations suffer from systematic
uncertainties. Therefore we suggest to use the Cauchy-Riemann equations, that
perform the analytical continuation without assuming global information on the
function in the entire complex plane, only in the region through which the
equations are solved. We use as example the quark propagator in Landau gauge
Quantum Chromodynamics, that is known from lattice and Dyson-Schwinger studies
in Euclidean space. The drawback of the method is the instability of the
Cauchy-Riemann equations to high-frequency noise, that makes difficult to
achieve good accuracy. We also point out a few curiosities related to the Wick
rotation.Comment: 12 pages in EPJ double-column format, 16 figures. This version: added
paragraph, two reference
Efficacy of oral appliance therapy in patients following uvulopalatopharyngoplasty failure
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149311/1/lio2256.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149311/2/lio2256_am.pd
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
Some New/Old Approaches to QCD
This is a talk delivered at the Meeting on Integrable Quantum Field Theories,
Villa Olmo and at STRINGS 1992, Rome, September 1992. I discuss some recent
attempts to revive two old ideas regarding an analytic approach to QCD-the
development of a string representation of the theory and the large N limit of
QCD.Comment: 20 page
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