10,729 research outputs found
Indicator systems - resource use in organic systems
A balanced use of resources within organic farming systems is required to maintain sustainable systems. Hence, it is essential to have tools that can assess the use of resources within the farming system and their impact on the environment. The range of tools that have been developed include those assessing local farm-scale issues together with those that assess impacts at the global scale. At the global scale assessments are usually made on the basis of a unit of product whereas at the local scale assessments can also be made on an area basis. In addition, the tools also assess a variety of issues, e.g. biodiversity, pollution potential, energy and water use. The level of detail required for the different assessment tools differs substantially; nevertheless it is essential that the indicator systems developed are based on sound knowledge, are acceptable to the farmers and can guide their future actions
Events in the life of a cocoon surrounding a light, collapsar jet
According to the collapsar model, gamma-ray bursts are thought to be produced
in shocks that occur after the relativistic jet has broken free from the
stellar envelope. If the mass density of the collimated outflow is less than
that of the stellar envelope, the jet will then be surrounded by a cocoon of
relativistic plasma. This material would itself be able to escape along the
direction of least resistance, which is likely to be the rotation axis of the
stellar progenitor, and accelerate in approximately the same way as an
impulsive fireball. We discuss how the properties of the stellar envelope have
a decisive effect on the appearance of a cocoon propagating through it. The
relativistic material that accumulated in the cocoon would have enough kinetic
energy to substantially alter the structure of the relativistic outflow, if not
in fact provide much of the observed explosive power. Shock waves within this
plasma can produce gamma-ray and X-ray transients, in addition to the standard
afterglow emission that would arise from the deceleration shock of the cocoon
fireball.Comment: 16 pages, 5 figures, slightly revised version, accepted for
publication in MNRA
Improving supply and phosphorous use efficiency in organic farming systems
Phosphorus (P) is an essential plant nutrient that needs to be managed carefully in organic systems so that crop yield and quality remain sustainable without contributing to environmental damage, particularly that associated with eutrophication. Under organic regulations, minimally processed rock phosphate (PR) can be used to amend low P fertility soils, although the solubility is extremely low at optimum soil pH for most crop growth (pH 6.5). This paper describes a project (PLINK) which aims to develop methods of improving P efficiency on organic farms, although the same approaches may also be applicable on conventional and low-input farms. The methodologies that the project is developing include the fermentation and composting of crop waste material with PR in order to solubilise P and make it more available to the crop. Some initial results are described here. In addition, the project will investigate the alteration of the rotation to include crops or varieties with high P uptake efficiency, or roots that possess acidifying properties which improve P availability for following crops
Darcy-Bénard-Bingham convection
The present paper is the first to consider Darcy–Bénard–Bingham convection. A Bingham fluid saturates a horizontal porous layer that is subjected to heating from below. It is shown that this simple extension to the classical Darcy–Bénard problem is linearly stable to small-amplitude disturbances but nevertheless admits strongly nonlinear convection. The Pascal model for a Bingham fluid occupying a porous medium is adopted, and this law is regularized in a frame-invariant manner to yield a set of two-dimensional governing equations that are then solved numerically using finite difference approximations. A weakly nonlinear theory of the regularized Pascal model is used to show that the onset of convection is via a fold bifurcation. Some parametric studies are performed to show that this non-linear onset of convection arises at increasing values of the Darcy–Rayleigh number as the Rees–Bingham number increases and that, for a fixed Rees–Bingham number, the wavenumber at which the rate of heat transfer is maximized increases with the Darcy–Rayleigh number
Developing a national dental education research strategy:priorities, barriers and enablers
Objectives: This study aimed to identify national dental education research (DER) priorities for the next 3-5 years and to identify barriers and enablers to DER. Setting: Scotland Participants: In this two-stage online questionnaire study we collected data with multiple dental professions (e.g. dentistry, dental nursing, dental hygiene) and stakeholder groups (e.g. learners, clinicians, educators, managers, researchers, academics). Eighty-five participants completed the Stage 1 qualitative questionnaire and 649 participants the Stage 2 quantitative questionnaire. Results: Eight themes were identified at Stage 1. Of the 24 DER priorities identified, the top three were: role of assessments in identifying competence; undergraduate curriculum prepares for practice; and promoting teamwork. Following exploratory factor analysis, the 24 items loaded onto four factors: teamwork and professionalism, measuring and enhancing performance, dental workforce issues, and curriculum integration and innovation. Barriers and enablers existed at multiple levels: individual, interpersonal, institutional structures and cultures, and technology. Conclusion: This priority setting exercise provides a necessary first step to developing a national DER strategy capturing multiple perspectives. Promoting DER requires improved resourcing alongside efforts to overcome peer stigma and lack of valuing and motivation
The Minimum Description Length Principle and Model Selection in Spectropolarimetry
It is shown that the two-part Minimum Description Length Principle can be
used to discriminate among different models that can explain a given observed
dataset. The description length is chosen to be the sum of the lengths of the
message needed to encode the model plus the message needed to encode the data
when the model is applied to the dataset. It is verified that the proposed
principle can efficiently distinguish the model that correctly fits the
observations while avoiding over-fitting. The capabilities of this criterion
are shown in two simple problems for the analysis of observed
spectropolarimetric signals. The first is the de-noising of observations with
the aid of the PCA technique. The second is the selection of the optimal number
of parameters in LTE inversions. We propose this criterion as a quantitative
approach for distinguising the most plausible model among a set of proposed
models. This quantity is very easy to implement as an additional output on the
existing inversion codes.Comment: Accepted for publication in the Astrophysical Journa
Are the hosts of VLBI selected radio-AGN different to those of radio-loud AGN?
Recent studies have found that radio-AGN selected by radio-loudness show
little difference in terms of their host galaxy properties when compared to
non-AGN galaxies of similar stellar mass and redshift. Using new 1.4~GHz VLBI
observations of the COSMOS field we find that approximately 49\% of
high-mass (M 10 M), high luminosity (L
10 W~Hz) radio-AGN possess a VLBI detected counterpart. These
objects show no discernible bias towards specific stellar masses, redshifts or
host properties other than what is shown by the radio-AGN population in
general. Radio-AGN that are detected in VLBI observations are not special, but
form a representative sample of the radio-loud AGN population.Comment: 6 pages, 4 figures, lette
Deepest Near-IR Surface Photometry of Galaxies in the Local Sphere of Influence
We present near-IR, deep (4 mag deeper than 2MASS) imaging of 56 Local Volume
galaxies. Global parameters such as total magnitudes and stellar masses have
been derived and the new near-IR data combined with existing 21cm and optical
B-band data. We present multiwavelength relations such as the HI mass-to-light
ratio and investigate the maximum total baryonic mass a galaxy can have.Comment: 4 pages, 3 figures, To be published in the proceedings of "Galaxies
in the Local Volume", ed. B. Koribalski, H. Jerje
The observable effects of a photospheric component on GRB's and XRF's prompt emission spectrum
A thermal radiative component is likely to accompany the first stages of the
prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We
analyze the effect of such a component on the observable spectrum, assuming
that the observable effects are due to a dissipation process occurring below or
near the thermal photosphere. We consider both the internal shock model and a
'slow heating' model as possible dissipation mechanisms. For comparable energy
densities in the thermal and the leptonic component, the dominant emission
mechanism is Compton scattering. This leads to a nearly flat energy spectrum
(\nu F_\nu \propto \nu^0) above the thermal peak at ~10-100 keV and below
10-100 MeV, for a wide range of optical depths 0.03 <~ \tau_{\gamma e} <~ 100,
regardless of the details of the dissipation mechanism or the strength of the
magnetic field. At lower energies steep slopes are expected, while above 100
MeV the spectrum depends on the details of the dissipation process. For higher
values of the optical depth, a Wien peak is formed at 100 keV - 1 MeV, and no
higher energy component exists. For any value of \tau_{\gamma e}, the number of
pairs produced does not exceed the baryon related electrons by a factor larger
than a few. We conclude that dissipation near the thermal photosphere can
naturally explain both the steep slopes observed at low energies and a flat
spectrum above 10 keV, thus providing an alternative scenario to the optically
thin synchrotron - SSC model.Comment: Discussion added on the results of Baring & Braby (2004); Accepted
for publication in Ap.
Evolution of size-dependent flowering in a variable environment: construction and analysis of a stochastic integral projection model
Understanding why individuals delay reproduction is a classic problem in evolutionary biology. In plants, the study of reproductive delays is complicated because growth and survival can be size and age dependent, individuals of the same size can grow by different amounts and there is temporal variation in the environment. We extend the recently developed integral projection approach to include size- and age-dependent demography and temporal variation. The technique is then applied to a long-term individually structured dataset for Carlina vulgaris, a monocarpic thistle. The parameterized model has excellent descriptive properties in terms of both the population size and the distributions of sizes within each age class. In Carlina, the probability of flowering depends on both plant size and age. We use the parameterized model to predict this relationship, using the evolutionarily stable strategy approach. Considering each year separately, we show that both the direction and the magnitude of selection on the flowering strategy vary from year to year. Provided the flowering strategy is constrained, so it cannot be a step function, the model accurately predicts the average size at flowering. Elasticity analysis is used to partition the size- and age-specific contributions to the stochastic growth rate, λs. We use λs to construct fitness landscapes and show how different forms of stochasticity influence its topography. We prove the existence of a unique stochastic growth rate, λs, which is independent of the initial population vector, and show that Tuljapurkar's perturbation analysis for log(λs) can be used to calculate elasticities
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