365 research outputs found
The orbital structure of a tidally induced bar
Orbits are the key building blocks of any density distribution and their
study helps us understand the kinematical structure and the evolution of
galaxies. Here we investigate orbits in a tidally induced bar of a dwarf
galaxy, using an -body simulation of an initially disky dwarf galaxy
orbiting a Milky Way-like host. After the first pericenter passage, a tidally
induced bar forms in the stellar component of the dwarf. The bar evolution is
different than in isolated galaxies and our analysis focuses on the period
before it buckles. We study the orbits in terms of their dominant frequencies,
which we calculate in a Cartesian coordinate frame rotating with the bar. Apart
from the well-known x orbits we find many other types, mostly with boxy
shapes of various degree of elongation. Some of them are also near-periodic,
admitting frequency ratios of 4/3, 3/2 and 5/3. The box orbits have various
degrees of vertical thickness but only a relatively small fraction of those
have banana (i.e. smile/frown) or infinity-symbol shapes in the edge-on view.
In the very center we also find orbits known from the potential of triaxial
ellipsoids. The elongation of the orbits grows with distance from the center of
the bar in agreement with the variation of the shape of the density
distribution. Our classification of orbits leads to the conclusion that more
than of them have boxy shapes, while only have shapes of
classical x orbits.Comment: 15 pages, 15 figures, accepted for publication in Ap
Tidally induced bars in dwarf galaxies on different orbits around a Milky Way-like host
Bars in galaxies may develop through a global instability or due to an
interaction with another system. We study bar formation in disky dwarf galaxies
orbiting a Milky Way-like galaxy. We employ -body simulations to study the
impact of initial orbital parameters: the size of the dwarf galaxy orbit and
the inclination of its disc with respect to the orbital plane. In all cases a
bar develops in the center of the dwarf during the first pericenter on its
orbit around the host. Between subsequent pericenter passages the bars are
stable, but at the pericenters they are usually weakened and shortened. The
initial properties and details of the further evolution of the bars depend
heavily on the orbital configuration. We find that for the exactly prograde
orientation, the strongest bar is formed for the intermediate-size orbit. On
the tighter orbit, the disc is too disturbed and stripped to form a strong bar.
On the wider orbit, the tidal interaction is too weak. The dependence on the
disc inclination is such that weaker bars form in more inclined discs. The bars
experience either a very weak buckling or none at all. We do not observe any
secular evolution, possibly because the dwarfs are perturbed at each pericenter
passage. The rotation speed of the bars can be classified as slow
(). We attribute this to the loss of a
significant fraction of the disc's rotation during the encounter with the host
galaxy.Comment: 17 pages, 14 figures, accepted to Ap
Formation of soliton trains in Bose-Einstein condensates by temporal Talbot effect
We study the recent observation of formation of matter-wave soliton trains in
Bose-Einstein condensates. We emphasize the role of the box-like confinement of
the Bose-Einstein condensate and find that there exist time intervals for the
opening the box that support the generation of real solitons. When the box-like
potential is switched off outside the existing time windows, the number of
peaks in a train changes resembling missing solitons observed in the
experiment. Our findings indicate that a new way of generating soliton trains
in condensates through the temporal, matter-wave Talbot effect is possible.Comment: 4 pages, 4 figures, new result
Adventures of a tidally induced bar
Using N-body simulations, we study the properties of a bar induced in a discy dwarf galaxy as a result of tidal interaction with the Milky Way. The bar forms at the first pericentre passage and survives until the end of the evolution at 10 Gyr. Fourier decomposition of the bar reveals that only even modes are significant and preserve a hierarchy so that the bar mode is always the strongest. They show a characteristic profile with a maximum, similar to simulated bars forming in isolated galaxies and observed bars in real galaxies. We adopt the maximum of the bar mode as a measure of the bar strength and we estimate the bar length by comparing the density profiles along the bar and perpendicular to it. The bar strength and the bar length decrease with time, mainly at pericentres, as a result of tidal torques acting at those times and not to secular evolution. The pattern speed of the bar varies significantly on a time-scale of 1 Gyr and is controlled by the orientation of the tidal torque from the Milky Way. The bar is never tidally locked, but we discover a hint of a 5/2 orbital resonance between the third and fourth pericentre passage. The speed of the bar decreases in the long run so that the bar changes from initially rather fast to slow in the later stages. The boxy/peanut shape is present for some time and its occurrence is preceded by a short period of buckling instability
Soliton trains in Bose-Fermi mixtures
We theoretically consider the formation of bright solitons in a mixture of
Bose and Fermi degenerate gases. While we assume the forces between atoms in a
pure Bose component to be effectively repulsive, their character can be changed
from repulsive to attractive in the presence of fermions provided the Bose and
Fermi gases attract each other strongly enough. In such a regime the Bose
component becomes a gas of effectively attractive atoms. Hence, generating
bright solitons in the bosonic gas is possible. Indeed, after a sudden increase
of the strength of attraction between bosons and fermions (realized by using a
Feshbach resonance technique or by firm radial squeezing of both samples)
soliton trains appear in the Bose-Fermi mixture.Comment: 4 pages, 4 figure
Statistical properties of one dimensional Bose gas
Monte Carlo method within, so called, classical fields approximation is
applied to one dimensional weakly interacting repulsive Bose gas trapped in a
harmonic potential. Equilibrium statistical properties of the condensate are
calculated within a canonical ensemble. We also calculate experimentally
relevant low order correlation functions of the whole gas
Superfluidity of the BEC at finite temperature
We use the classical fields approximation to study a translational flow of
the condensate with respect to the thermal cloud in a weakly interacting Bose
gas. We study both, subcritical and supercritical relative velocity cases and
analyze in detail a state of stationary flow which is reached in the dynamics.
This state corresponds to the thermal equilibrium, which is characterized by
the relative velocity of the condensate and the thermal cloud. The
superfluidity manifests itself in the existence of many thermal equilibria
varying in (the value of this velocity) the relative velocity between the
condensate and the thermal cloud. We pay a particular attention to excitation
spectra in a phonon as well as in a particle regime. Finally, we introduce a
measure of the amount of the superfluid fraction in a weakly interacting Bose
gas, allowing for the precise distinction between the superfluid and the
condensed fractions in a single and consistent framework.Comment: 8 pages, 5 figure
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Inter-granular effects at high magnetic fields of cuprate and iron chalcogenide superconducting materials
The weak links effects are one of the main challenges for effective power applications of high temperature superconducting materials. Studies of these effects help for their better understanding and subsequent improvement. An overview analysis of the intergranular properties of cuprate (Y0.8Ca0.2Ba2Cu3O7-δ) and iron-based chalcogenide (FeSe0.5Te0.5) polycrystalline samples was carried out, by means of series of electro-transport experiments at different magnetic fields. The temperature evolution of the Josephson coupling and intrinsic superconductivity effects for the both systems was constructed. The FeSe0.5Te0.5 compound shows very stable and superior behavior compared to Y0.8Ca0.2BCO up to the highest magnetic fields (14T) used. We have explored FeSe0.5Te0.5 Josephson weak links influence (as a non-linear process) over the resistive transition using different AC current amplitudes and applying the sensitive AC transport third harmonics technique
hp-HGS strategy for inverse 3D DC resistivity logging measurement simulations
In this paper we present a twin adaptive strategy hp-HGS for solving inverse problems related to 3D DC borehole resistivity measurement simulations. The term "simulation of measurements" is widely used by the geophysical community. A quantity of interest, voltage, is measured at a receiver electrode located in the logging instrument. We use the self-adaptive goal-oriented hp-Finite Element Method (hp-FEM) computer simulations of the process of measurements in deviated wells (when the angle between the borehole and formation layers are < 90 deg). We also employ the hierarchical genetic search (HGS) algorithm to solve the inverse problem. Each individual in the population represents a single configuration of the formation layers. The evaluation of the individual is performed by solving the direct problem by means of the hp-FEM algorithm and by comparison with measured logging curve. We conclude the paper with some discussion on the parallelization of the algorithm. © 2012 Published by Elsevier Ltd
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