364 research outputs found
Screened thermonuclear reactions and predictive stellar evolution of detached double-lined eclipsing binaries
The low energy fusion cross sections of charged-particle nuclear reactions
(and the respective reaction rates) in stellar plasmas are enhanced due to
plasma screening effects. We study the impact of those effects on predictive
stellar evolution simulations for detached double-lined eclipsing binaries. We
follow the evolution of binary systems (pre-main sequence or main sequence
stars) with precisely determined radii and masses from 1.1Mo to 23Mo (from
their birth until their present state). The results indicate that all the
discrepancies between the screened and unscreened models (in terms of
luminosity, stellar radius, and effective temperature) are within the
observational uncertainties. Moreover, no nucleosynthetic or compositional
variation was found due to screening corrections. Therefore all thermonuclear
screening effects on the charged-particle nuclear reactions that occur in the
binary stars considered in this work (from their birth until their present
state) can be totally disregarded. In other words, all relevant
charged-particle nuclear reactions can be safely assumed to take place in a
vacuum, thus simplifying and accelerating the simulation processes.Comment: 5 RevTex pages,no figures. Accepted for publication in Phys.Rev.
Stationary states of a nonlinear Schrödinger lattice with a harmonic trap
We study a discrete nonlinear Schrödinger lattice with a parabolic trapping potential. The model, describing, e.g., an array of repulsive Bose-Einstein condensate droplets confined in the wells of an optical lattice, is analytically and numerically investigated. Starting from the linear limit of the problem, we use global bifurcation theory to rigorously prove that – in the discrete regime – all linear states lead to nonlinear generalizations thereof, which assume the form of a chain of discrete dark solitons (as the density increases). The stability of the ensuing nonlinear states is studied and it is found that the ground state is stable, while the excited states feature a chain of stability/instability bands. We illustrate the mechanisms under which discreteness destabilizes the dark-soliton configurations, which become stable only in the continuum regime. Continuation from the anti-continuum limit is also considered, and a rich bifurcation structure is revealed
Intrinsic Energy Localization through Discrete Gap Breathers in One-Dimensional Diatomic Granular Crystals
We present a systematic study of the existence and stability of discrete
breathers that are spatially localized in the bulk of a one-dimensional chain
of compressed elastic beads that interact via Hertzian contact. The chain is
diatomic, consisting of a periodic arrangement of heavy and light spherical
particles. We examine two families of discrete gap breathers: (1) an unstable
discrete gap breather that is centered on a heavy particle and characterized by
a symmetric spatial energy profile and (2) a potentially stable discrete gap
breather that is centered on a light particle and is characterized by an
asymmetric spatial energy profile. We investigate their existence, structure,
and stability throughout the band gap of the linear spectrum and classify them
into four regimes: a regime near the lower optical band edge of the linear
spectrum, a moderately discrete regime, a strongly discrete regime that lies
deep within the band gap of the linearized version of the system, and a regime
near the upper acoustic band edge. We contrast discrete breathers in anharmonic
FPU-type diatomic chains with those in diatomic granular crystals, which have a
tensionless interaction potential between adjacent particles, and highlight in
that the asymmetric nature of the latter interaction potential may lead to a
form of hybrid bulk-surface localized solutions
Dark solitons in cigar-shaped Bose-Einstein condensates in double-well potentials
We study the statics and dynamics of dark solitons in a cigar-shaped
Bose-Einstein condensate confined in a double-well potential. Using a
mean-field model with a non-cubic nonlinearity, appropriate to describe the
dimensionality crossover regime from one to three dimensional, we obtain
branches of solutions in the form of single- and multiple-dark soliton states,
and study their bifurcations and stability. It is demonstrated that there exist
dark soliton states which do not have a linear counterpart and we highlight the
role of anomalous modes in the excitation spectra. Particularly, we show that
anomalous mode eigenfrequencies are closely connected to the characteristic
soliton frequencies as found from the solitons' equations of motion, and how
anomalous modes are related to the emergence of instabilities. We also analyze
in detail the role of the height of the barrier in the double well setting,
which may lead to instabilities or decouple multiple dark soliton states.Comment: 35 pages, 12 figure
Dynamics of interacting dark soliton stripes
In the present work we examine the statics and dynamics of multiple parallel
dark soliton stripes in a two-dimensional Bose-Einstein condensate. Our
principal goal is to study the effect of the interaction between the stripes on
the transverse instability of the individual stripes. We use a recently
developed adiabatic invariant formulation to derive a quasi-analytical
prediction for the stripe equilibrium position and for the Bogoliubov-de Gennes
spectrum of excitations of stationary stripes. The cases of two-, three- and
four-stripe states are studied in detail. We subsequently test our predictions
against numerical simulations of the full two-dimensional Gross-Pitaevskii
equation. We find that the number of unstable eigenmodes increases as the
number of stripes increases due to (unstable) relative motions between the
stripes. Their corresponding growth rates do not significantly change, although
for large chemical potentials, the larger the stripe number, the larger the
maximal instability growth rate. The instability induced dynamics of multiple
stripe states and their decay into vortices are also investigated.Comment: 13 pages, 11 figure
Multiple atomic dark solitons in cigar-shaped Bose-Einstein condensates
We consider the stability and dynamics of multiple dark solitons in
cigar-shaped Bose-Einstein condensates (BECs). Our study is motivated by the
fact that multiple matter-wave dark solitons may naturally form in such
settings as per our recent work [Phys. Rev. Lett. 101, 130401 (2008)]. First,
we study the dark soliton interactions and show that the dynamics of
well-separated solitons (i.e., ones that undergo a collision with relatively
low velocities) can be analyzed by means of particle-like equations of motion.
The latter take into regard the repulsion between solitons (via an effective
repulsive potential) and the confinement and dimensionality of the system (via
an effective parabolic trap for each soliton). Next, based on the fact that
stationary, well-separated dark multi-soliton states emerge as a nonlinear
continuation of the appropriate excited eigensates of the quantum harmonic
oscillator, we use a Bogoliubov-de Gennes analysis to systematically study the
stability of such structures. We find that for a sufficiently large number of
atoms, multiple soliton states may be dynamically stable, while for a small
number of atoms, we predict a dynamical instability emerging from resonance
effects between the eigenfrequencies of the soliton modes and the intrinsic
excitation frequencies of the condensate. Finally we present experimental
realizations of multi-soliton states including a three-soliton state consisting
of two solitons oscillating around a stationary one.Comment: 17 pages, 11 figure
IGF-IR cooperates with ERα to inhibit breast cancer cell aggressiveness by regulating the expression and localisation of ECM molecules
IGF-IR is highly associated with the behaviour of breast cancer cells. In ERα-positive breast cancer, IGF-IR is present at high levels. In clinical practice, prolonged treatment with anti-estrogen agents results in resistance to the therapy with activation of alternative signaling pathways. Receptor Tyrosine Kinases, and especially IGF-IR, have crucial roles in these processes. Here, we report a nodal role of IGF-IR in the regulation of ERα-positive breast cancer cell aggressiveness and the regulation of expression levels of several extracellular matrix molecules. In particular, activation of IGF-IR, but not EGFR, in MCF-7 breast cancer cells results in the reduction of specific matrix metalloproteinases and their inhibitors. In contrast, IGF-IR inhibition leads to the depletion by endocytosis of syndecan-4. Global important changes in cell adhesion receptors, which include integrins and syndecan-4 triggered by IGF-IR inhibition, regulate adhesion and invasion. Cell function assays that were performed in MCF-7 cells as well as their ERα-suppressed counterparts indicate that ER status is a major determinant of IGF-IR regulatory role on cell adhesion and invasion. The strong inhibitory role of IGF-IR on breast cancer cells aggressiveness for which E2-ERα signaling pathway seems to be essential, highlights IGF-IR as a major molecular target for novel therapeutic strategies
Dark-in-Bright Solitons in Bose-Einstein Condensates with Attractive Interactions
We demonstrate a possibility to generate localized states in effectively
one-dimensional Bose-Einstein condensates with a negative scattering length in
the form of a dark soliton in the presence of an optical lattice (OL) and/or a
parabolic magnetic trap. We connect such structures with twisted localized
modes (TLMs) that were previously found in the discrete nonlinear
Schr{\"o}dinger equation. Families of these structures are found as functions
of the OL strength, tightness of the magnetic trap, and chemical potential, and
their stability regions are identified. Stable bound states of two TLMs are
also found. In the case when the TLMs are unstable, their evolution is
investigated by means of direct simulations, demonstrating that they transform
into large-amplitude fundamental solitons. An analytical approach is also
developed, showing that two or several fundamental solitons, with the phase
shift between adjacent ones, may form stable bound states, with
parameters quite close to those of the TLMs revealed by simulations. TLM
structures are found numerically and explained analytically also in the case
when the OL is absent, the condensate being confined only by the magnetic trap.Comment: 13 pages, 7 figures, New Journal of Physics (in press
Stability of dark solitons in a Bose-Einstein condensate trapped in an optical lattice
We investigate the stability of dark solitons (DSs) in an effectively
one-dimensional Bose-Einstein condensate in the presence of the magnetic
parabolic trap and an optical lattice (OL). The analysis is based on both the
full Gross-Pitaevskii equation and its tight-binding approximation counterpart
(discrete nonlinear Schr{\"o}dinger equation). We find that DSs are subject to
weak instabilities with an onset of instability mainly governed by the period
and amplitude of the OL. The instability, if present, sets in at large times
and it is characterized by quasi-periodic oscillations of the DS about the
minimum of the parabolic trap.Comment: Typo fixed in Eq. (1): cos^2 -> sin^
Vortices in a Bose-Einstein condensate confined by an optical lattice
We investigate the dynamics of vortices in repulsive Bose-Einstein
condensates in the presence of an optical lattice (OL) and a parabolic magnetic
trap. The dynamics is sensitive to the phase of the OL potential relative to
the magnetic trap, and depends less on the OL strength. For the cosinusoidal OL
potential, a local minimum is generated at the trap's center, creating a stable
equilibrium for the vortex, while in the case of the sinusoidal potential, the
vortex is expelled from the center, demonstrating spiral motion. Cases where
the vortex is created far from the trap's center are also studied, revealing
slow outward-spiraling drift. Numerical results are explained in an analytical
form by means of a variational approximation. Finally, motivated by a discrete
model (which is tantamount to the case of the strong OL lattice), we present a
novel type of vortex consisting of two pairs of anti-phase solitons.Comment: 10 pages, 6 figure
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