1,462 research outputs found
The group law on the tropical Hesse pencil
We show that the addition of points on the tropical Hesse curve can be
realized via the intersection with a tropical line. Then the addition formula
for the tropical Hesse curve is reduced from those for the level-three theta
functions through the ultradiscretization procedure. A tropical analogue of the
Hessian group, the group of linear automorphisms acting on the Hesse pencil, is
also investigated; it is shown that the dihedral group of degree three is the
group of linear automorphisms acting on the tropical Hesse pencil.Comment: 17 pages, 1 figure, submitted to Special Issue of the Journal
Mathematics and Computers in Simulation on "Nonlinear Waves: Computation and
Theory
Assessment of a sequential phase extraction procedure for uranium-series isotope analysis of soils and sediments
The study of uranium-series (U-series) isotopes in soil and sediment materials has been proposed to quantify rates and timescales of soil production and sediment transport. Previous works have studied bulk soil or sediment material, which is a complex assemblage of primary and secondary minerals and organic compounds. However, the approach relies on the fractionation between U-series isotopes in primary minerals since they were liberated from the parent rock via weathering. In addition, secondary minerals and organic compounds have their own isotopic compositions such that the composition of the bulk material may not reflect that of primary minerals. Hence, there is a need for a sample preparation procedure that allows the isolation of primary minerals in soil or fluvial sediment samples. In this study, a sequential extraction procedure to separate primary minerals from soils and sediments was assessed. The procedure was applied to standard rock sample powders (TML-3 and BCR-2) to test whether it introduced any artefactual radioactive disequilibrium. A new step was introduced to remove the clay-sized fraction (\u3c2 \u3e”m). Significant amounts (5â14%) of U and Th were removed from the rock standards during the procedure. No significant alteration in (234U/238U) and (230Th/238U) activity ratios of the rock standards occurred during the procedure. Aliquots of soil sample were subjected to the sequential extraction process to test how each step modifies the uranium-series activity ratios and mineralogy. Although no secondary minerals were detected in the unleached soil aliquots, the sequential leaching process removed up to 17% of U and Th and modified their activity ratios by up to 3%. The modification of the activity ratios poses a demand for careful means to avoid redistribution of isotopes back to the residual phase during phase extraction
The role of external broadcasting in a closed political system
This article investigates the role and impact of external broadcasting (radio and television) on a closed political system, through the example of the two post-war German states: the West German Federal Republic of Germany (FRG) and the East German German Democratic Republic (GDR). The aim is to debunk myths about the influence of external broadcasting on the events that led to German reunification in 1990. The study follows a historical approach and discusses what role external media played during the years of a divided Germany. The findings are based on several historical sources, research reports from the 1950s and 1960s and over 100 biographical interviews with former residents of the German Democratic Republic (GDR). The article analyses the impact of external broadcasting on citizens and the political elite in times of crisis as well as during everyday life
Scaling Laws and Transient Times in 3He Induced Nuclear Fission
Fission excitation functions of compound nuclei in a mass region where shell
effects are expected to be very strong are shown to scale exactly according to
the transition state prediction once these shell effects are accounted for. The
fact that no deviations from the transition state method have been observed
within the experimentally investigated excitation energy regime allows one to
assign an upper limit for the transient time of 10 zs.Comment: 7 pages, TeX type, psfig, submitted to Phys. Rev. C, also available
at http://csa5.lbl.gov/moretto/ps/he3_paper.p
First INTEGRAL Observations of Eight Persistent Neutron Star Low Mass X-ray Binaries
Early results from the INTEGRAL Core Program, for a sample of eight
persistently bright neutron star low mass X-ray binaries in the energy range
from 5 keV to 200 keV are presented. It is shown that INTEGRAL efficiently
detects sources and that spectra may be obtained up to several hundreds of keV
by combining data from three of the four INTEGRAL instruments: JEM-X, IBIS and
SPI. For the source GX 17+2 it is shown that the spectrum extends well above
100 keV with a flattening of the spectrum above 30 keV. This might suggest a
non-thermal comptonisation emission, but uncertainties in the current data
reduction and background determination do not allow firm conclusions to be
drawn.Comment: 5 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Epistemic and social scripts in computer-supported collaborative learning
Collaborative learning in computer-supported learning environments typically means that learners work on tasks together, discussing their individual perspectives via text-based media or videoconferencing, and consequently acquire knowledge. Collaborative learning, however, is often sub-optimal with respect to how learners work on the concepts that are supposed to be learned and how learners interact with each other. One possibility to improve collaborative learning environments is to conceptualize epistemic scripts, which specify how learners work on a given task, and social scripts, which structure how learners interact with each other. In this contribution, two studies will be reported that investigated the effects of epistemic and social scripts in a text-based computer-supported learning environment and in a videoconferencing learning environment in order to foster the individual acquisition of knowledge. In each study the factors âepistemic scriptâ and âsocial scriptâ have been independently varied in a 2Ă2-factorial design. 182 university students of Educational Science participated in these two studies. Results of both studies show that social scripts can be substantially beneficial with respect to the individual acquisition of knowledge, whereas epistemic scripts apparently do not to lead to the expected effects
Stellar Evolutionary Effects on the Abundances of PAH and SN-Condensed Dust in Galaxies
Spectral and photometric observations of nearby galaxies show a correlation
between the strength of their mid-IR aromatic features, attributed to PAH
molecules, and their metal abundance, leading to a deficiency of these features
in low-metallicity galaxies. In this paper, we suggest that the observed
correlation represents a trend of PAH abundance with galactic age, reflecting
the delayed injection of carbon dust into the ISM by AGB stars in the final
post-AGB phase of their evolution. AGB stars are the primary sources of PAHs
and carbon dust in galaxies, and recycle their ejecta back to the interstellar
medium only after a few hundred million years of evolution on the main
sequence. In contrast, more massive stars that explode as Type II supernovae
inject their metals and dust almost instantaneously after their formation. We
first determined the PAH abundance in galaxies by constructing detailed models
of UV-to-radio SED of galaxies that estimate the contribution of dust in
PAH-free HII regions, and PAHs and dust from photodissociation regions, to the
IR emission. All model components: the galaxies' stellar content, properties of
their HII regions, and their ionizing and non-ionizing radiation fields and
dust abundances, are constrained by their observed multiwavelength spectrum.
After determining the PAH and dust abundances in 35 nearby galaxies using our
SED model, we use a chemical evolution model to show that the delayed injection
of carbon dust by AGB stars provides a natural explanation to the dependence of
the PAH content in galaxies with metallicity. We also show that larger dust
particles giving rise to the far-IR emission follow a distinct evolutionary
trend closely related to the injection of dust by massive stars into the ISM.Comment: ApJ, 69 pages, 46 figures, Accepte
The potential of marginal coastal nursery habitats for the conservation of a culturally important Caribbean marine species
Aim: Identifying the potential of marginal habitats for species conservation is of key importance when their core high-quality habitats are under substantial disturbances and threats. However, there is currently a knowledge gap on how useful marine marginal habitats may be for conserving endangered marine species. Here, we investigate the potential of groundwater-fed coastal areas for the conservation of the queen conch, an economically and culturally important marine gastropod.
Location: The inlet of Xel-Ha, typical of groundwater-fed coastal areas widely distributed along the Yucatan Peninsula coast in Mexico and partially protected by a network of marine protected areas.
Methods: We tracked 66 queen conchs (Lobatus gigas) using acoustic telemetry over a period of 3.5 years. We investigated for ontogenetic niche shift using a network analysis and by modelling their growth.
Results: The queen conchs exhibited the same ontogenetic niche shift required to complete their life cycle in this marginal habitat as they do in offshore core habitats. A total of 33 individuals departed the inlet and migrated from shallow groundwater-affected nursery grounds to deeper marine habitats more suitable for breeding aggregation.
Main conclusions: As the broad-scale movement behaviour of queen conch in this inlet is similar to that observed on the overfished core habitats, our findings suggest that groundwater-fed coastal areas should be included in conservation planning for an effective management of this species within a network of marine protected areas
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