3,772 research outputs found
Effect of the Canting of Local Anisotropy Axes on Ground-State Properties of a Ferrimagnetic Chain with Regularly Alternating Ising and Heisenberg Spins
The effect of the canting of local anisotropy axes on the ground-state phase
diagram and magnetization of a ferrimagnetic chain with regularly alternating
Ising and Heisenberg spins is exactly examined in an arbitrarily oriented
magnetic field. It is shown that individual contributions of Ising and
Heisenberg spins to the total magnetization basically depend on the spatial
orientation of the magnetic field and the canting angle between two different
local anisotropy axes of the Ising spins.Comment: 3 pages, 3 figure
Dissipative maps at the chaos threshold: Numerical results for the single-site map
We numerically study, at the edge of chaos, the behaviour of the sibgle-site
map , where is the map parameter.Comment: 8 pages with 4 figures, submitted to Physica
A Link Between the Semi-Major Axis of Extrasolar Gas Giant Planets and Stellar Metallicity
The fact that most extrasolar planets found to date are orbiting metal-rich
stars lends credence to the core accretion mechanism of gas giant planet
formation over its competitor, the disc instability mechanism. However, the
core accretion mechanism is not refined to the point of explaining orbital
parameters such as their unexpected semi-major axes and eccentricities. We
propose a model, which correlates the metallicity of the host star with the
original semi-major axis of its most massive planet, prior to migration,
considering that the core accretion scenario governs giant gas planet
formation. The model predicts that the optimum regions for planetary formation
shift inward as stellar metallicity decreases, providing an explanation for the
observed absence of long period planets in metal-poor stars. We compare our
predictions with the available data on extrasolar planets for stars with masses
similar to the mass of the Sun. A fitting procedure produces an estimate of
what we define as the Zero Age Planetary Orbit (ZAPO) curve as a function of
the metallicity of the star. The model also hints that the lack of planets
circling metal-poor stars may be partly caused by an enhanced destruction
probability during the migration process, since the planets lie initially
closer to the central stars.Comment: Nature of the replacement: According to recent simulations, the
temperature profile, T, is more adequately reproduced by beta = 1 rather than
beta = 2. We have introduced a distance scale factor that solves the very
fast drop of T for low metallicity and introduces naturally the inferior
distance limit of our ZAPO. Under this modification all the fitting process
was altere
Tetrapedia diversipes in Restricted Areas in Bahia, Brazil (Hymenoptera: Apidae)
Tetrapedia species are solitary bees which collect floral oils, being restricted to tropical regions of the Americas. Information on forms of nesting has been little researched in the literature, requiring studies on the diversity and variability of species to obtain better management and conservation strategies for their populations. Morphometry is a efficient technique and has been used to detect variation and for identification of species of bees in order to detect changes in quantitative traits within and among populations of bees. This study aimed to compare the variability of the population of Tetrapedia diversipes in artificial nests located in orchards and their surroundings (other fruit) of acerola in a restricted area of the Reconcavo region of Bahia, Brazil. Right wings were extracted from 155 individuals of the T. diversipes species, to perform the morphometric analysis. In conventional morphometry, 9 variables contributed significantly to the sexual dimorphism in the study areas (α = 0.05). The geometric morphometric analysis revealed low gene flow in populations of T. diversipes demonstrating loss of genetic diversity, requiring proper management of this bee for its conservation and maintenance of the associated flora
Dust-trapping Rossby vortices in protoplanetary disks
One of the most challenging steps in planet formation theory is the one
leading to the formation of planetesimals of kilometre size. A promising
scenario involves the existence of vortices able to concentrate a large amount
of dust and grains in their centres. Up to now this scenario has been studied
mostly in 2D razor thin disks. A 3D study including, simultaneously, the
formation and resulting dust concentration of the vortices with vertical
settling, was still missing. The Rossby wave instability self-consistently
forms 3D vortices, which have the unique quality of presenting a large scale
vertical velocity in their centre. Here we aim to study how this newly
discovered effect can alter the dynamic evolution of the dust. We perform
global 3D simulations of the RWI in a radially and vertically stratified disk
using the code MPI-AMRVAC. After the growth phase of the instability, the gas
and solid phases are modelled by a bi-fluid approach, where the dust is
considered as a fluid without pressure. Both the drag force of the gas on the
dust and the back-reaction of the dust on the gas are included. Multiple grain
sizes from 1mm to 5cm are used with a constant density distribution. We obtain
in a short timescale a high concentration of the largest grains in the
vortices. Indeed, in 3 rotations the dust-to-gas density ratio grows from 10^-2
to unity leading to a concentration of mass up to that of Mars in one vortex.
The presence of the radial drift is also at the origin of a dust pile-up at the
radius of the vortices. Lastly, the vertical velocity of the gas in the vortex
causes the sedimentation process to be reversed, the mm size dust is lifted and
higher concentrations are obtained in the upper layer than in the mid-plane.Comment: Accepted for publication in Astronomy and Astrophysic
Signs In Time: The Hypothesis Of Social Time For A Cultural Semiotic Analysis
http://www.ester.ee/record=b4482490*es
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