6,403 research outputs found
Numerical simulations of multiple scattering of the mode by flux tubes
We use numerial simulations to study the absorption and phase shift of
surface-gravity waves caused by groups of magnetic flux tubes. The dependence
of the scattering coefficients with the distance between the tubes and their
positions is analyzed for several cases with two or three flux tubes embedded
in a quiet Sun atmosphere. The results are compared with those obtained
neglecting completely or partially multiple scattering effects. We show that
multiple scattering has a significant impact on the absorption measurements and
tends to reduce the phase shift. We also consider more general cases of
ensembles of randomly distributed flux tubes, and we have evaluated the effects
on the scattering measurements of changing the number of tubes included in the
bundle and the average distance between flux tubes. We find that for the
longest wavelength incoming waves multiple scattering enhances the absorption,
and its efficiency increases with the number of flux tubes and the reduction of
the distance between them.Comment: Accepted for publication in The Astrophysical Journa
Evaluation of the capability of local helioseismology to discern between monolithic and spaghetti sunspot models
The helioseismic properties of the wave scattering generated by monolithic
and spaghetti sunspots are analyzed by means of numerical simulations. In these
computations, an incident f or p1 mode travels through the sunspot model, which
produces absorption and phase shift of the waves. The scattering is studied by
inspecting the wavefield, computing travel-time shifts, and performing
Fourier-Hankel analysis. The comparison between the results obtained for both
sunspot models reveals that the differences in the absorption coefficient can
be detected above noise level. The spaghetti model produces an steep increase
of the phase shift with the degree of the mode at short wavelengths, while
mode-mixing is more efficient for the monolithic model. These results provide a
clue for what to look for in solar observations to discern the constitution of
sunspots between the proposed monolithic and spaghetti models.Comment: Accepted for publication in The Astrophysical Journa
Helioseismic holography of simulated sunspots: magnetic and thermal contributions to travel times
Wave propagation through sunspots involves conversion between waves of
acoustic and magnetic character. In addition, the thermal structure of sunspots
is very different than that of the quiet Sun. As a consequence, the
interpretation of local helioseismic measurements of sunspots has long been a
challenge. With the aim of understanding these measurements, we carry out
numerical simulations of wave propagation through sunspots. Helioseismic
holography measurements made from the resulting simulated wavefields show
qualitative agreement with observations of real sunspots. We use additional
numerical experiments to determine, separately, the influence of the thermal
structure of the sunspot and the direct effect of the sunspot magnetic field.
We use the ray approximation to show that the travel-time shifts in the thermal
(non-magnetic) sunspot model are primarily produced by changes in the wave path
due to the Wilson depression rather than variations in the wave speed. This
shows that inversions for the subsurface structure of sunspots must account for
local changes in the density. In some ranges of horizontal phase speed and
frequency there is agreement (within the noise level in the simulations)
between the travel times measured in the full magnetic sunspot model and the
thermal model. If this conclusion proves to be robust for a wide range of
models, it would suggest a path towards inversions for sunspot structure.Comment: Accepted for publication in The Astrophysical Journa
Constraints on the braneworld from compact stars
According to the braneworld idea, ordinary matter is confined on a
3-dimensional space (brane) that is embedded in a higher-dimensional space-time
where gravity propagates. In this work, after reviewing the limits coming from
general relativity, finiteness of pressure and causality on the brane, we
derive observational constraints on the braneworld parameters from the
existence of stable compact stars. The analysis is carried out by solving
numerically the brane-modified Tolman-Oppenheimer-Volkoff equations, using
different representative equations of state to describe matter in the star
interior. The cases of normal dense matter, pure quark matter and hybrid matter
are considered.Comment: 13 pages, 11 figures, 2 tables; new EoS considered, references and
comments adde
Scattering of the f-mode by small magnetic flux elements from observations and numerical simulations
The scattering of f-modes by magnetic tubes is analyzed using
three-dimensional numerical simulations. An f-mode wave packet is propagated
through a solar atmosphere embedded with three different flux tube models which
differ in radius and total magnetic flux. A quiet Sun simulation without a tube
present is also performed as a reference. Waves are excited inside the flux
tube and propagate along the field lines, and jacket modes are generated in the
surroundings of the flux tube, carrying 40% as much energy as the tube modes.
The resulting scattered wave is mainly an f-mode composed of a mixture of m=0
and m=+/-1 modes. The amplitude of the scattered wave approximately scales with
the magnetic flux. A small amount of power is scattered into the p_1-mode. We
have evaluated the absorption and phase shift from a Fourier-Hankel
decomposition of the photospheric vertical velocities. They are compared with
the results obtained from the emsemble average of 3400 small magnetic elements
observed in high-resolution MDI Doppler datacubes. The comparison shows that
the observed dependence of the phase shift with wavenumber can be matched
reasonably well with the simulated flux tube model. The observed variation of
the phase-shifts with the azimuthal order appears to depend on details of
the ensemble averaging, including possible motions of the magnetic elements and
asymmetrically shaped elements.Comment: Accepted for publication in The Astrophysical Journa
Mass-radius relation for magnetized strange quark stars
We review the stability of magnetized strange quark matter (MSQM) within the
phenomenological MIT bag model, taking into account the variation of the
relevant input parameters, namely, the strange quark mass, baryon density,
magnetic field and bag parameter. A comparison with magnetized asymmetric quark
matter in -equilibrium as well as with strange quark matter (SQM) is
presented. We obtain that the energy per baryon for MSQM decreases as the
magnetic field increases, and its minimum value at vanishing pressure is lower
than the value found for SQM, which implies that MSQM is more stable than
non-magnetized SQM. The mass-radius relation for magnetized strange quark stars
is also obtained in this framework.Comment: 7 pages, 6 figures. To be published in the Proceedings of 4th
International Workshop on Relativistic Astrophysical and Astronomy IWARA0
Kinetic energy change with doping upon superfluid condensation in high temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases
upon superfluid condensation (the change DEkin is positive). Here we show that
in the high critical temperature superconductor Bi-2212, DEkin crosses over
from a fully compatible conventional BCS behavior (DEkin>0) to an
unconventional behavior (DEkin<0) as the free carrier density decreases. If a
single mechanism is responsible for superconductivity across the whole phase
diagram of high critical temperature superconductors, this mechanism should
allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure
Tunable Holstein model with cold polar molecules
We show that an ensemble of polar molecules trapped in an optical lattice can
be considered as a controllable open quantum system. The coupling between
collective rotational excitations and the motion of the molecules in the
lattice potential can be controlled by varying the strength and orientation of
an external DC electric field as well as the intensity of the trapping laser.
The system can be described by a generalized Holstein Hamiltonian with tunable
parameters and can be used as a quantum simulator of excitation energy transfer
and polaron phenomena. We show that the character of excitation energy transfer
can be modified by tuning experimental parameters.Comment: 5 pages, 3 figures (accepted in as a Rapid Communication in
Phys.Rev.A
Neutrinos and the matter-antimatter asymmetry in the Universe
The discovery of neutrino oscillations provides a solid evidence for nonzero
neutrino masses and leptonic mixing. The fact that neutrino masses are so tiny
constitutes a puzzling problem in particle physics. From the theoretical
viewpoint, the smallness of neutrino masses can be elegantly explained through
the seesaw mechanism. Another challenging issue for particle physics and
cosmology is the explanation of the matter-antimatter asymmetry observed in
Nature. Among the viable mechanisms, leptogenesis is a simple and
well-motivated framework. In this talk we briefly review these aspects, making
emphasis on the possibility of linking neutrino physics to the cosmological
baryon asymmetry originated from leptogenesis.Comment: 8 pages, 1 table, 1 figure; Based on talk given at the Symposium
STARS2011, 1 - 4 May 2011, Havana, Cuba; to be published in the Proceeding
Gravitational baryogenesis in Gauss-Bonnet braneworld cosmology
The mechanism of gravitational baryogenesis, based on the CPT-violating
gravitational interaction between the derivative of the Ricci scalar curvature
and the baryon-number current, is investigated in the context of the
Gauss-Bonnet braneworld cosmology. We study the constraints on the fundamental
five-dimensional gravity scale, the effective scale of B-violation and the
decoupling temperature, for the above mechanism to generate an acceptable
baryon asymmetry during the radiation-dominated era. The scenario of
gravitational leptogenesis, where the lepton-number violating interactions are
associated with the neutrino mass seesaw operator, is also considered.Comment: 8 pages, 3 figures, final version to appear in Phys. Rev.
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