16,534 research outputs found
Relativistic Klein-Gordon-Maxwell multistream model for quantum plasmas
A multistream model for spinless electrons in a relativistic quantum plasma
is introduced by means of a suitable fluid-like version of the
Klein-Gordon-Maxwell system. The one and two-stream cases are treated in
detail. A new linear instability condition for two-stream quantum plasmas is
obtained, generalizing the previously known non-relativistic results. In both
the one and two-stream cases, steady-state solutions reduce the model to a set
of coupled nonlinear ordinary differential equations, which can be numerically
solved, yielding a manifold of nonlinear periodic and soliton structures. The
validity conditions for the applicability of the model are addressed
Circumstellar Disks revealed by / Flux Variation Gradients
The variability of young stellar objects (YSO) changes their brightness and
color preventing a proper classification in traditional color-color and color
magnitude diagrams. We have explored the feasibility of the flux variation
gradient (FVG) method for YSOs, using and band monitoring data of the
star forming region RCW\,38 obtained at the University Observatory Bochum in
Chile. Simultaneous multi-epoch flux measurements follow a linear relation
for almost all YSOs with large variability
amplitude. The slope gives the mean color temperature of
the varying component. Because is hotter than the dust sublimation
temperature, we have tentatively assigned it to stellar variations. If the
gradient does not meet the origin of the flux-flux diagram, an additional non-
or less-varying component may be required. If the variability amplitude is
larger at the shorter wavelength, e.g. , this component is cooler
than the star (e.g. a circumstellar disk); vice versa, if , the
component is hotter like a scattering halo or even a companion star. We here
present examples of two YSOs, where the FVG implies the presence of a
circumstellar disk; this finding is consistent with additional data at and
. One YSO shows a clear -band excess in the color-color diagram,
while the significance of a -excess in the other YSO depends on the
measurement epoch. Disentangling the contributions of star and disk it turns
out that the two YSOs have huge variability amplitudes (\,mag). The
FVG analysis is a powerful complementary tool to analyze the varying
components of YSOs and worth further exploration of monitoring data at other
wavelengths.Comment: 5 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Generalized Hamiltonian structures for Ermakov systems
We construct Poisson structures for Ermakov systems, using the Ermakov
invariant as the Hamiltonian. Two classes of Poisson structures are obtained,
one of them degenerate, in which case we derive the Casimir functions. In some
situations, the existence of Casimir functions can give rise to superintegrable
Ermakov systems. Finally, we characterize the cases where linearization of the
equations of motion is possible
Quantum Antiferromagnetism in Quasicrystals
The antiferromagnetic Heisenberg model is studied on a two-dimensional
bipartite quasiperiodic lattice. The distribution of local staggered magnetic
moments is determined on finite square approximants with up to 1393 sites,
using the Stochastic Series Expansion Quantum Monte Carlo method. A non-trivial
inhomogeneous ground state is found. For a given local coordination number, the
values of the magnetic moments are spread out, reflecting the fact that no two
sites in a quasicrystal are identical. A hierarchical structure in the values
of the moments is observed which arises from the self-similarity of the
quasiperiodic lattice. Furthermore, the computed spin structure factor shows
antiferromagnetic modulations that can be measured in neutron scattering and
nuclear magnetic resonance experiments.
This generic model is a first step towards understanding magnetic
quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.Comment: RevTex, 4 pages with 5 figure
On the linearization of the generalized Ermakov systems
A linearization procedure is proposed for Ermakov systems with frequency
depending on dynamic variables. The procedure applies to a wide class of
generalized Ermakov systems which are linearizable in a manner similar to that
applicable to usual Ermakov systems. The Kepler--Ermakov systems belong into
this category but others, more generic, systems are also included
Visualization of oxygen distribution patterns caused by coral and algae.
Planar optodes were used to visualize oxygen distribution patterns associated with a coral reef associated green algae (Chaetomorpha sp.) and a hermatypic coral (Favia sp.) separately, as standalone organisms, and placed in close proximity mimicking coral-algal interactions. Oxygen patterns were assessed in light and dark conditions and under varying flow regimes. The images show discrete high oxygen concentration regions above the organisms during lighted periods and low oxygen in the dark. Size and orientation of these areas were dependent on flow regime. For corals and algae in close proximity the 2D optodes show areas of extremely low oxygen concentration at the interaction interfaces under both dark (18.4 ± 7.7 µmol O2 L(- 1)) and daylight (97.9 ± 27.5 µmol O2 L(- 1)) conditions. These images present the first two-dimensional visualization of oxygen gradients generated by benthic reef algae and corals under varying flow conditions and provide a 2D depiction of previously observed hypoxic zones at coral algae interfaces. This approach allows for visualization of locally confined, distinctive alterations of oxygen concentrations facilitated by benthic organisms and provides compelling evidence for hypoxic conditions at coral-algae interaction zones
On the Hamiltonian structure of Ermakov systems
A canonical Hamiltonian formalism is derived for a class of Ermakov systems
specified by several different frequency functions. This class of systems
comprises all known cases of Hamiltonian Ermakov systems and can always be
reduced to quadratures. The Hamiltonian structure is explored to find exact
solutions for the Calogero system and for a noncentral potential with dynamic
symmetry. Some generalizations of these systems possessing exact solutions are
also identified and solved
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