10,882 research outputs found
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
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
Internal Migration and Regional Population Dynamics in Europe: Estonia Case Study
Estonia has experienced a long-lasting and strong influence of international migration on regional population growth. Post-war immigrants account for about 36 per cent of the total population, and are concentrated in larger cities of Northern Estonia. Regionally, the relative proportions of the native-born and immigrant origin sub-populations are important for the understanding of population change and internal migration flows in the 1980-1990s.
In Estonia, the quality of migration data requires careful assessment. The preservation of Soviet-type record-keeping has reduced data quality in the 1990s, already low, and use of the data should keep data quality problems in mind. Otherwise, false conclusions can be reached.
To describe internal migration patterns, it has proved technically feasible and very useful to disaggregate the county population into rural and urban components, and correspondingly, the migration flows into four directions (urban-urban, urban-rural, rural-urban and rural-rural).
During the 1980s the pattern of population growth and internal migration has changed in Estonia. Reflecting the turnaround in long-term population processes, migration development reached the advanced stage with more or less regionally balanced in- and out-migration flows and decreasing importance of net migration. Accordingly, to understand current trends and patterns, explanations must be sought from the 1980s which has served a starting point for the present trends rather than from the period of economic transition in the 1990s.
As a part of the turnaround, the century-long persistent rural depopulation has come to an end and the moderate growth has started reflecting natural population increase as well as deurbanization. In the 1980s two developments have occurred in parallel: migratory increase of rural population led by a deurbanizing native-born population, and continued urban population growth as a result of the population momentum of pre-transition immigrants. In future decades, the urban deconcentration will probably be the underlying trend in Estonia. In Estonia, noticeable proportion of territory and population is located in islands. However, the island population does not show any systematic difference in the type of internal migration. Particularly, the depopulation of island populations, observed in several comparable European cases, is not occurring.
Each life-course stage was found to have its specific migration pattern, more stable than the pattern for the total population. In many cases the changes of internal migration are determined by the change in the proportion of population in different life-course stages. Additionally, the life-course approach has been useful in demonstrating the features of the present Estonian internal migration pattern which appear closer to the countries of comparable in demographic development, more or less regardless of the significant differences in the level of economic development. Among life-course groups, in Estonia the older working age population was characterized by the strongest deurbanization intensities in 1995. The same group has also undergone the largest modification of migration pattern during the economic transition (1987-1995)
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
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
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.
Anthropic reasoning in multiverse cosmology and string theory
Anthropic arguments in multiverse cosmology and string theory rely on the
weak anthropic principle (WAP). We show that the principle, though ultimately a
tautology, is nevertheless ambiguous. It can be reformulated in one of two
unambiguous ways, which we refer to as WAP_1 and WAP_2. We show that WAP_2, the
version most commonly used in anthropic reasoning, makes no physical
predictions unless supplemented by a further assumption of "typicality", and we
argue that this assumption is both misguided and unjustified. WAP_1, however,
requires no such supplementation; it directly implies that any theory that
assigns a non-zero probability to our universe predicts that we will observe
our universe with probability one. We argue, therefore, that WAP_1 is
preferable, and note that it has the benefit of avoiding the inductive
overreach characteristic of much anthropic reasoning.Comment: 7 pages. Expanded discussion of selection effects and some minor
clarifications, as publishe
The HST Large Program on Omega Centauri. I. Multiple stellar populations at the bottom of the main sequence probed in NIR-Optical
As part of a large investigation with Hubble Space Telescope to study the
faintest stars within the globular cluster Omega Centauri, in this work we
present early results on the multiplicity of its main sequence (MS) stars,
based on deep optical and near-infrared observations. By using appropriate
color-magnitude diagrams we have identified, for the first time, the two main
stellar populations I, and II along the entire MS, from the turn-off towards
the hydrogen-burning limit. We have compared the observations with suitable
synthetic spectra of MS stars and conclude that the two MSs are consistent with
stellar populations with different metallicity, helium, and light-element
abundance. Specifically, MS-I corresponds to a metal-poor stellar population
([Fe/H]~-1.7) with Y~ 0.25 and [O/Fe]~0.30. The MS-II hosts helium-rich
(Y~0.37-0.40) stars with metallicity ranging from [Fe/H]~-1.7 to -1.4. Below
the MS knee (mF160W~19.5, our photometry reveals that each of the two main MSs
hosts stellar subpopulations with different oxygen abundances, with very O-poor
stars ([O/Fe]~-0.5) populating the MS-II. Such a complexity has never been
observed in previous studies of M-dwarfs in globular clusters. A few months
before the lunch of the James Webb Space Telescope, these results demonstrate
the power of optical and near-infrared photometry in the study of multiple
stellar populations in globular clusters.Comment: 13 pages, 9 figures, accepted for publication in MNRA
A Model for the Moving `Wisps' in the Crab Nebula
I propose that the moving `wisps' near the center of the Crab Nebula result
from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the
shocked pulsar wind. Recent observations suggest that the wisps trace out
circular wavefronts in this plane, expanding radially at speeds approximately
less than c/3. Instabilities could develop if there is sufficient velocity
shear between a faster-moving equatorial zone and a slower moving shocked
pulsar wind at higher latitudes. The development of shear could be related to
the existence of a neutral sheet -- with weak magnetic field -- in the
equatorial zone, and could also be related to a recent suggestion by Begelman
that the magnetic field in the Crab pulsar wind is much stronger than had been
thought. I show that plausible conditions could lead to the growth of
instabilities at the radii and speeds observed, and that their nonlinear
development could lead to the appearance of sharp wisplike features.Comment: 7 pages; 3 postscript figures; LaTex, uses emulateapj.sty; to Appear
in the Astrophysical Journal, Feb. 20, 1999, Vol. 51
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