1,767 research outputs found
N, P and K budgets for crop rotations on nine organic farms in the UK
On organic farms, where the importation of materials to build/maintain soil fertility is restricted, it is important that a balance between inputs and outputs of nutrients is achieved to ensure both short-term productivity and long-term sustainability. This paper considers different approaches to nutrient budgeting on organic farms and evaluates the sources of bias in the measurements and/or estimates of the nutrient inputs and outputs. The paper collates 88 nutrient budgets compiled at the farm scale in 9 temperate countries. All the nitrogen (N) budgets showed an N surplus (average 83.2 kg N ha-1 year-1). The efficiency of N use, defined as outputs/inputs, was highest (0.9) and lowest (0.2) in arable and beef systems respectively. The phosphorus (P) and potassium (K) budgets showed both surpluses and deficits (average 3.6 kg P ha-1 year-1, 14.2 kg K ha-1 year-1) with horticultural systems showing large surpluses resulting from purchased manure. The estimation of N fixation and quantities of nutrients in purchased manures may introduce significant errors in nutrient budgets. Overall, the data illustrate the diversity of management systems in place on organic farms, and suggest that used together with soil analysis, nutrient budgets are a useful tool for improving the long-term sustainability of organic systems
Postbuckled stability of panels with torsional buckling
The panel analysis and optimization code VICONOPT, based on exact strip theory, is
utilized to investigate the postbuckling stability of a stiffened aerospace panel in a torsional
buckled state. The paper shows that the postbuckling characteristics of a panel buckling in
a torsional mode has similarity to the postbuckling behavior of a panel with a skin initiated
mode and a panel initiated mode. The postbuckled stiffness of the torsional mode is similar
to the skin mode in terms of load versus end shortening and is similar to the panel
postbuckling behavior in terms of load versus out-of-plane deflection. If the panel has
stiffeners of more than one size then there are multiple torsional modes. For panel design it
is suggested that small stiffener buckling, i.e., in a torsional mode, can have postbuckling
stability with regard to the growth of the out-of-plane deflection. If the large stiffeners
initiate the buckling then there is no postbuckling reserve of strength. This has implications
for design of such panels as mass could be saved if allowance is made for small stiffener
buckling in the optimization process
Roche tomography of cataclysmic variables: I. artefacts and techniques
Roche tomography is a technique used for imaging the Roche-lobe filling
secondary stars in cataclysmic variables (CVs). In order to interpret Roche
tomograms correctly, one must determine whether features in the reconstruction
are real, or due to statistical or systematic errors. We explore the effects of
systematic errors using reconstructions of simulated datasets and show that
systematic errors result in characteristic distortions of the final
reconstructions that can be identified and corrected. In addition, we present a
new method of estimating statistical errors on tomographic reconstructions
using a Monte-Carlo bootstrapping algorithm and show this method to be much
more reliable than Monte-Carlo methods which `jiggle' the data points in
accordance with the size of their error bars.Comment: 11 pages, 8 figures. Accepted for publication in MNRA
Nuclear Astrophysics in Storage Rings
Nuclear reaction cross sections are usually very small in typical
astrophysical environments. It has been one of the major challenges of
experimental nuclear astrophysics to assess the magnitude of these cross
sections in the laboratory. For a successful experiment high luminosity beams
are needed. Increasing the target width, one also increases the reaction
yields. But, this is of limited use due to multiple scattering in the target.
Storage rings are a very good way to overcome these difficulties. In principle,
they can be tuned to large luminosities, and have the advantage of crossing the
interaction region many times per second (typically one million/s),
compensating low density internal gas targets, or low reaction rates in
beam-beam collisions. Storage rings are also ideal tools for precise
measurements of masses and beta-decay lifetimes of nuclei of relevance for
astrophysics.Comment: 14 pages, LaTeX, figures available upon reques
Noncolliding Squared Bessel Processes
We consider a particle system of the squared Bessel processes with index conditioned never to collide with each other, in which if
the origin is assumed to be reflecting. When the number of particles is finite,
we prove for any fixed initial configuration that this noncolliding diffusion
process is determinantal in the sense that any multitime correlation function
is given by a determinant with a continuous kernel called the correlation
kernel. When the number of particles is infinite, we give sufficient conditions
for initial configurations so that the system is well defined. There the
process with an infinite number of particles is determinantal and the
correlation kernel is expressed using an entire function represented by the
Weierstrass canonical product, whose zeros on the positive part of the real
axis are given by the particle-positions in the initial configuration. From the
class of infinite-particle initial configurations satisfying our conditions, we
report one example in detail, which is a fixed configuration such that every
point of the square of positive zero of the Bessel function is
occupied by one particle. The process starting from this initial configuration
shows a relaxation phenomenon converging to the stationary process, which is
determinantal with the extended Bessel kernel, in the long-term limit.Comment: v3: LaTeX2e, 26 pages, no figure, corrections made for publication in
J. Stat. Phy
Nuclear Astrophysics in Rare Isotope Facilities
Nuclear reactions in stars are difficult to measure directly in the
laboratory at the small astrophysical energies. In recent years indirect
methods with rare isotopes have been developed and applied to extract
low-energy astrophysical cross sections.Comment: Invited talk (parallel section) at the Int. Conf. Nucleus-Nucleus
Collisions (NN2009), Beijing, China, August 16-21, 2009. To appear in Nucl.
Phys.
The JigCell Model Builder: A Spreadsheet Interface for Creating Biochemical Reaction Network Models
SDSS J150722.30+523039.8: a cataclysmic variable formed directly from a detached white dwarf/brown dwarf binary?
We present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J150722.30+523039.8 (hereafter SDSS J1507). This system has an orbital period of 66.61 min, placing it below the observed âperiod minimumâ for cataclysmic variables. We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by Ï2 minimization. We obtain a mass ratio of q= 0.0623 ± 0.0007 and an orbital inclination forumlaâ . The primary mass is Mw= 0.90 ± 0.01âMâ. The secondary mass and radius are found to be Mr= 0.056 ± 0.001âMâ and Rr= 0.096 ± 0.001âRâ, respectively. We find a distance to the system of 160 ± 10âpc. The secondary star in SDSS J1507 has a mass substantially below the hydrogen burning limit, making it the second confirmed substellar donor in a cataclysmic variable.
The very short orbital period of SDSS J1507 is readily explained if the secondary star is nuclearly evolved, or if SDSS J1507 formed directly from a detached white dwarf/brown dwarf binary. Given the lack of any visible contribution from the secondary star, the very low secondary mass and the low HeâIλ6678/Hα emission-line ratio, we argue that SDSS J1507 probably formed directly from a detached white dwarf/brown dwarf binary. If confirmed, SDSS J1507 will be the first such system identified. The implications for binary star evolution, the brown dwarf desert and the common envelope phase are discussed
Modeling DNA Structure, Elasticity and Deformations at the Base-pair Level
We present a generic model for DNA at the base-pair level. We use a variant
of the Gay-Berne potential to represent the stacking energy between neighboring
base-pairs. The sugar-phosphate backbones are taken into account by semi-rigid
harmonic springs with a non-zero spring length. The competition of these two
interactions and the introduction of a simple geometrical constraint leads to a
stacked right-handed B-DNA-like conformation. The mapping of the presented
model to the Marko-Siggia and the Stack-of-Plates model enables us to optimize
the free model parameters so as to reproduce the experimentally known
observables such as persistence lengths, mean and mean squared base-pair step
parameters. For the optimized model parameters we measured the critical force
where the transition from B- to S-DNA occurs to be approximately . We
observe an overstretched S-DNA conformation with highly inclined bases that
partially preserves the stacking of successive base-pairs.Comment: 15 pages, 25 figures. submitted to PR
Habitable Zones of Host Stars During the Post-MS Phase
A star will become brighter and brighter with stellar evolution, and the
distance of its habitable zone will become farther and farther. Some planets
outside the habitable zone of a host star during the main sequence phase may
enter the habitable zone of the host star during other evolutionary phases. A
terrestrial planet within the habitable zone of its host star is generally
thought to be suited to life existence. Furthermore, a rocky moon around a
giant planet may be also suited to life survive, provided that the planet-moon
system is within the habitable zone of its host star. Using Eggleton's code and
the boundary flux of habitable zone, we calculate the habitable zone of our
Solar after the main sequence phase. It is found that Mars' orbit and Jupiter's
orbit will enter the habitable zone of Solar during the subgiant branch phase
and the red giant branch phase, respectively. And the orbit of Saturn will
enter the habitable zone of Solar during the He-burning phase for about 137
million years. Life is unlikely at any time on Saturn, as it is a giant gaseous
planet. However, Titan, the rocky moon of Saturn, may be suitable for
biological evolution and become another Earth during that time. For low-mass
stars, there are similar habitable zones during the He-burning phase as our
Solar, because there are similar core masses and luminosities for these stars
during that phase.Comment: 6 pages, 7 figures. Accepted by Ap & S
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