8,582 research outputs found
Vortex lattice stability and phase coherence in three-dimensional rapidly rotating Bose condensates
We establish the general equations of motion for the modes of a vortex
lattice in a rapidly rotating Bose-Einstein condensate in three dimensions,
taking into account the elastic energy of the lattice and the vortex line
bending energy. As in two dimensions, the vortex lattice supports Tkachenko and
gapped sound modes. In contrast, in three dimensions the Tkachenko mode
frequency at long wavelengths becomes linear in the wavevector for any
propagation direction out of the transverse plane. We compute the correlation
functions of the vortex displacements and the superfluid order parameter for a
homogeneous Bose gas of bounded extent in the axial direction. At zero
temperature the vortex displacement correlations are convergent at large
separation, but at finite temperatures, they grow with separation. The growth
of the vortex displacements should lead to observable melting of vortex
lattices at higher temperatures and somewhat lower particle number and faster
rotation than in current experiments. At zero temperature a system of large
extent in the axial direction maintains long range order-parameter correlations
for large separation, but at finite temperatures the correlations decay with
separation.Comment: 10 pages, 2 figures, Changes include the addition of the particle
density - vortex density coupling and the correct value of the shear modulu
Tkachenko modes of vortex lattices in rapidly rotating Bose-Einstein condensates
We calculate the in-plane modes of the vortex lattice in a rotating Bose
condensate from the Thomas-Fermi to the mean-field quantum Hall regimes. The
Tkachenko mode frequency goes from linear in the wavevector, , for lattice
rotational velocities, , much smaller than the lowest sound wave
frequency in a finite system, to quadratic in in the opposite limit. The
system also supports an inertial mode of frequency . The
calculated frequencies are in good agreement with recent observations of
Tkachenko modes at JILA, and provide evidence for the decrease in the shear
modulus of the vortex lattice at rapid rotation.Comment: 4 pages, 2 figure
Grid Search in Stellar Parameters: a software for spectrum analysis of single stars and binary systems
The currently operating space missions, as well as those that will be
launched in the near future, (will) deliver high-quality data for millions of
stellar objects. Since the majority of stellar astrophysical applications still
(at least partly) rely on spectroscopic data, an efficient tool for the
analysis of medium- to high-resolution spectroscopy is needed. We aim at
developing an efficient software package for the analysis of medium- to
high-resolution spectroscopy of single stars and those in binary systems. The
major requirements are that the code has a high performance, represents the
state-of-the-art analysis tool, and provides accurate determinations of
atmospheric parameters and chemical compositions for different types of stars.
We use the method of atmosphere models and spectrum synthesis, which is one of
the most commonly used approaches for the analysis of stellar spectra. Our Grid
Search in Stellar Parameters (GSSP) code makes use of the OpenMPI
implementation, which makes it possible to run in parallel mode. The method is
first tested on the simulated data and is then applied to the spectra of real
stellar objects. The majority of test runs on the simulated data were
successful in the sense that we could recover the initially assumed sets of
atmospheric parameters. We experimentally find the limits in signal-to-noise
ratios of the input spectra, below which the final set of parameters gets
significantly affected by the noise. Application of the GSSP package to the
spectra of three Kepler stars, KIC11285625, KIC6352430, and KIC4931738, was
also largely successful. We found an overall agreement of the final sets of the
fundamental parameters with the original studies. For KIC6352430, we found that
dependence of the light dilution factor on wavelength cannot be ignored, as it
has significant impact on the determination of the atmospheric parameters of
this binary system.Comment: 19 pages, 6 figures, 4 tables, 2 appendices one of which includes
detailed description of input and output files. Accepted for publication in
Astronomy & Astrophysi
Probing high-mass stellar evolutionary models with binary stars
Mass discrepancy is one of the problems that is pending a solution in
(massive) binary star research field. The problem is often solved by
introducing an additional near core mixing into evolutionary models, which
brings theoretical masses of individual stellar components into an agreement
with the dynamical ones. In the present study, we perform a detailed analysis
of two massive binary systems, V380 Cyg and Sigma Sco, to provide an
independent, asteroseismic measurement of the overshoot parameter, and to test
state-of-the-art stellar evolution models.Comment: 5 pages, 1 figure, 3 tables; proceedings of the IAU Symposium 307
held in Geneva in June 201
Vortex lattices in rapidly rotating Bose-Einstein condensates: modes and correlation functions
After delineating the physical regimes which vortex lattices encounter in
rotating Bose-Einstein condensates as the rotation rate, , increases,
we derive the normal modes of the vortex lattice in two dimensions at zero
temperature. Taking into account effects of the finite compressibility, we find
an inertial mode of frequency , and a primarily transverse
Tkachenko mode, whose frequency goes from being linear in the wave vector in
the slowly rotating regime, where is small compared with the lowest
compressional mode frequency, to quadratic in the wave vector in the opposite
limit. We calculate the correlation functions of vortex displacements and
phase, density and superfluid velocities, and find that the zero-point
excitations of the soft quadratic Tkachenko modes lead in a large system to a
loss of long range phase correlations, growing logarithmically with distance,
and hence lead to a fragmented state at zero temperature. The vortex positional
ordering is preserved at zero temperature, but the thermally excited Tkachenko
modes cause the relative positional fluctuations to grow logarithmically with
separation at finite temperature. The superfluid density, defined in terms of
the transverse velocity autocorrelation function, vanishes at all temperatures.
Finally we construct the long wavelength single particle Green's function in
the rotating system and calculate the condensate depletion as a function of
temperature.Comment: 11 pages Latex, no figure
Tkachenko modes as sources of quasiperiodic pulsar spin variations
We study the long wavelength shear modes (Tkachenko waves) of triangular
lattices of singly quantized vortices in neutron star interiors taking into
account the mutual friction between the superfluid and the normal fluid and the
shear viscosity of the normal fluid. The set of Tkachenko modes that propagate
in the plane orthogonal to the spin vector are weakly damped if the coupling
between the superfluid and normal fluid is small. In strong coupling, their
oscillation frequencies are lower and are undamped for small and moderate shear
viscosities. The periods of these modes are consistent with the observed
~100-1000 day variations in spin of PSR 1828-11.Comment: 7 pages, 3 figures, uses RevTex, v2: added discussion/references,
matches published versio
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