5,037 research outputs found
Symmetry reduction, integrability and reconstruction in k-symplectic field theory
We investigate the reduction process of a k-symplectic field theory whose
Lagrangian is invariant under a symmetry group. We give explicit coordinate
expressions of the resulting reduced partial differential equations, the
so-called Lagrange-Poincare field equations. We discuss two issues about
reconstructing a solution from a given solution of the reduced equations. The
first one is an interpretation of the integrability conditions, in terms of the
curvatures of some connections. The second includes the introduction of the
concept of a k-connection to provide a reconstruction method. We show that an
invariant Lagrangian, under suitable regularity conditions, defines a
`mechanical' k-connection.Comment: 37 page
Correlations in the QPO Frequencies of Low Mass X-Ray Binaries and the Relativistic Precession Model
A remarkable correlation between the centroid frequencies of quasi periodic
oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries,
has been recently discovered by Psaltis, Belloni and van der Klis (1999). This
correlation extends over nearly 3 decades in frequency and encompasses both
neutron star and black hole candidate systems. We discuss this result in the
light of the relativistic precession model, which has been proposed to
interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron
star low mass X-ray binaries of the Atoll and Z classes. Unlike other models
the relativistic precession model does not require the compact object to be a
neutron star and can be applied to black hole candidates as well. We show that
the predictions of the relativistic precession model match both the value and
dependence of the correlation to a very good accuracy without resorting to
additional assumptions.Comment: To appear in ApJ Letters. AASTEX Latex v. 5.0, 1 figure not include
Conducta social dels drogaaddictes. Causes del comportament delictiu. Es respecten els drets humans?
Aspects of the Unitarized Soft Multipomeron Approach in DIS and Diffraction
We study in detail the main features of the unitarized Regge model (CFKS),
recently proposed to describe the small- domain. It takes into account a
two-component description with two types of unitarized contributions: one is
the multiple pomeron exchange contribution, interacting with the large dipole
configurations, and the other one consists of a unitarized dipole cross
section, describing the interaction with the small size dipoles. We analyze the
ratio between soft and hard pieces as a function of the virtuality, and also
compare the resulting dipole cross section to that from the saturation model.
Diffraction dissociation is also considered, showing the scaling violations in
diffractive DIS and estimating the corresponding logarithmic slope.Comment: 14 pages, 5 postscript figures. Version to be published in Eur. Phys.
J.
Tuning the electronic, photophysical and charge transfer properties of small D-A molecules based on Thienopyrazine-terthienyls by changing the donor fragment: A DFT study
Indexación: Scopus.Four acceptor-donor organic conjugated molecules based on thieno[3,4-b]pyrazine-terthienyls were analyzed in order to explore the effect of the donor substituent on their molecular structures, electronic and optical properties. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations were carried out employing the B3LYP hybrid functional in combination with the 6-31G(d,p) basis set. The results suggests that the addition of electron-donating substituents to the conjugated molecules can diminish their energy gap value, which is beneficial to the photon harvesting. The lowest-lying absorption spectra of compounds substituted with electron donor groups exhibited a red-shift and a high oscillation factor compared with the unsubstituted molecule. Additionally, the ionization potential (IP), electron affinity (EA), reorganization energy (λ) and open-circuit voltage (Voc) of the molecules were evaluated. According to these values, the molecules show good photovoltaic properties, and efficient charge transfer for hole and electron and balanced charges.https://scielo.conicyt.cl/scielo.php?script=sci_arttext&pid=S0717-97072017000303637&lng=en&nrm=iso&tlng=e
Relativistic precession around rotating neutron stars: Effects due to frame-dragging and stellar oblateness
General relativity predicts that a rotating body produces a frame-dragging
(or Lense-Thirring) effect: the orbital plane of a test particle in a
non-equatorial orbit precesses about the body's symmetry axis. In this paper we
compute the precession frequencies of circular orbits around rapidly rotating
neutron stars for a variety of masses and equations of state. The precession
frequencies computed are expressed as numerical functions of the orbital
frequency observed at infinity. The post-Newtonian expansion of the exact
precession formula is examined to identify the relative magnitudes of the
precession caused by the Lense-Thirring effect, the usual Newtonian quadrupole
effect and relativistic corrections. The first post-Newtonian correction to the
Newtonian quadrupole precession is derived in the limit of slow rotation. We
show that the post-Newtonian precession formula is a good approximation to the
exact precession close to the neutron star in the slow rotation limit (up to
\sim 400 Hz in the present context).
The results are applied to recent RXTE observations of neutron star low-mass
X-ray binaries, which display kHz quasi-periodic oscillations and, within the
framework of beat frequency models, allow the measurement of both the neutron
star spin frequency and the Keplerian frequency of the innermost ring of matter
in the accretion disk around it. For a wide range of realistic equations of
state, we find that the predicted precession frequency of this ring is close to
one half of the low-frequency (\sim 20 - 35 Hz) quasi-periodic oscillations
seen in several Atoll sources.Comment: 35 pages including 10 figures and 6 tables. To appear in the
Astrophysical Journa
Differential Rotation in Neutron Stars: Magnetic Braking and Viscous Damping
Diffferentially rotating stars can support significantly more mass in
equilibrium than nonrotating or uniformly rotating stars, according to general
relativity. The remnant of a binary neutron star merger may give rise to such a
``hypermassive'' object. While such a star may be dynamically stable against
gravitational collapse and bar formation, the radial stabilization due to
differential rotation is likely to be temporary. Magnetic braking and viscosity
combine to drive the star to uniform rotation, even if the seed magnetic field
and the viscosity are small. This process inevitably leads to delayed collapse,
which will be accompanied by a delayed gravitational wave burst and, possibly,
a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the
braking of differential rotation by magnetic fields and viscosity. The star is
idealized as a differentially rotating, infinite cylinder consisting of a
homogeneous, incompressible conducting gas. We solve analytically the simplest
case in which the gas has no viscosity and the star resides in an exterior
vacuum. We treat numerically cases in which the gas has internal viscosity and
the star is embedded in an exterior, low-density, conducting medium. Our
evolution calculations are presented to stimulate more realistic MHD
simulations in full 3+1 general relativity. They serve to identify some of the
key physical and numerical parameters, scaling behavior and competing
timescales that characterize this important process.Comment: 11 pages. To appear in ApJ (November 20, 2000
On attributes of a Rotating Neutron star with a Hyperon core
We study the effect of rotation on global properties of neutron star with a
hyperon core in an effective chiral model with varying nucleon effective mass
within a mean field approach. The resulting gross properties of the rotating
compact star sequences are then compared and analyzed with other theoretical
predictions and observations from neutron stars. The maximum mass of the
compact star predicted by the model lies in the range at
Kepler frequency , which is consistant with recent observation of
high mass stars thereby reflecting the sensitivity of the underlying nucleon
effective mass in the dense matter EoS. We also discuss the implications of the
experimental constraints from the flow data from heavy-ion collisions on the
global properties of the rotating neutron stars.Comment: 11 Pages, 10 Figures and 2 Table
On spontaneous scalarization
We study in the physical frame the phenomenon of spontaneous scalarization
that occurs in scalar-tensor theories of gravity for compact objects. We
discuss the fact that the phenomenon occurs exactly in the regime where the
Newtonian analysis indicates it should not. Finally we discuss the way the
phenomenon depends on the equation of state used to describe the nuclear
matter.Comment: 41 pages, RevTex, 10 ps figures, submitted to Phys. Rev.
The Cauchy problem of scalar-tensor theories of gravity
The 3+1 formulation of scalar-tensor theories of gravity (STT) is obtained in
the physical (Jordan) frame departing from the 4+0 covariant field equations.
Contrary to the common belief (folklore), the new system of ADM-like equations
shows that the Cauchy problem of STT is well formulated (in the sense that the
whole system of evolution equations is of first order in the time-derivative).
This is the first step towards a full first order (in time and space)
formulation from which a subsequent hyperbolicity analysis (a well-posedness
determination) can be performed. Several gauge (lapse and shift) conditions are
considered and implemented for STT. In particular, a generalization of the
harmonic gauge for STT allows us to prove the well posedness of the STT using a
second order analysis which is very similar to the one used in general
relativity. Some spacetimes of astrophysical and cosmological interest are
considered as specific applications. Several appendices complement the ideas of
the main part of the paper.Comment: 29 pages Revtex; typos corrected; references added and updated; a
shorter version of this paper was published in Classical and Quantum Gravit
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