225 research outputs found
Equilibrium solutions of relativistic rotating stars with mixed poloidal and toroidal magnetic fields
Stationary and axisymmetric solutions of relativistic rotating stars with
strong mixed poloidal and toroidal magnetic fields are obtained numerically.
Because of the mixed components of the magnetic field, the underlying
stationary and axisymmetric spacetimes are no longer circular. These
configurations are computed from the full set of the Einstein-Maxwell
equations, Maxwell's equations and from first integrals and integrability
conditions of the magnetohydrodynamic equilibrium equations. After a brief
introduction of the formulation of the problem, we present the first results
for highly deformed magnetized rotating compact stars.Comment: 7 pages, to appear in PRD rapid communicatio
R-mode oscillations of rapidly rotating Newtonian stars - A new numerical scheme and its application to the spin evolution of neutron stars
We have developed a new numerical scheme to solve r-mode oscillations of {\it
rapidly rotating polytropic stars} in Newtonian gravity. In this scheme, Euler
perturbations of the density, three components of the velocity are treated as
four unknown quantities together with the oscillation frequency. For the basic
equations of oscillations, the compatibility equations are used instead of the
linearized equations of motion.
By using this scheme, we have solved the classical r-mode oscillations of
rotational equilibrium sequences of polytropes with the polytropic indices and 1.5 for and 4 modes. Here is the rank of the
spherical harmonics . These results have been applied to investigate
evolution of uniformly rotating hot young neutron stars by considering the
effect of gravitational radiation and viscosity. We have found that the maximum
angular velocities of neutron stars are around 10-20% of the Keplerian angular
velocity irrespective of the softness of matter. This confirms the results
obtained from the analysis of r-modes with the slow rotation approximation
employed by many authors.Comment: LaTeX 12 pages with 19 figures, to be published in PR
Numerical approach for high precision 3-D relativistic star models
A multi-domain spectral method for computing very high precision 3-D stellar
models is presented. The boundary of each domain is chosen in order to coincide
with a physical discontinuity (e.g. the star's surface). In addition, a
regularization procedure is introduced to deal with the infinite derivatives on
the boundary that may appear in the density field when stiff equations of state
are used. Consequently all the physical fields are smooth functions on each
domain and the spectral method is absolutely free of any Gibbs phenomenon,
which yields to a very high precision. The power of this method is demonstrated
by direct comparison with analytical solutions such as MacLaurin spheroids and
Roche ellipsoids. The relative numerical error reveals to be of the order of
. This approach has been developed for the study of relativistic
inspiralling binaries. It may be applied to a wider class of astrophysical
problems such as the study of relativistic rotating stars too.Comment: Minor changes, Phys. Rev. D in pres
SOFC durability against standby and shutdown cycling
To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H2S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail
Gravitational signals emitted by a point mass orbiting a neutron star: a perturbative approach
We compute the energy spectra of the gravitational signals emitted when a
pointlike mass moves on a closed orbit around a non rotating neutron star,
inducing a perturbation of its gravitational field and its internal structure.
The Einstein equations and the hydrodynamical equations are perturbed and
numerically integrated in the frequency domain. The results are compared with
the energy spectra computed by the quadrupole formalism which assumes that both
masses are pointlike, and accounts only for the radiation emitted because the
orbital motion produces a time dependent quadrupole moment. The results of our
perturbative approach show that, in general, the quadrupole formalism
overestimates the amount of emitted radiation, especially when the two masses
are close. However, if the pointlike mass is allowed to move on an orbit so
tight that the keplerian orbital frequency resonates with the frequency of the
fundamental quasi-normal mode of the star (2w_K=w_f), this mode can be excited
and the emitted radiation can be considerably larger than that computed by the
quadrupole approach.Comment: 36 pages, 7 figures, submimtted to Phys. Rev.
Electronic states and quantum transport in double-wall carbon nanotubes
Electronic states and transport properties of double-wall carbon nanotubes
without impurities are studied in a systematic manner. It is revealed that
scattering in the bulk is negligible and the number of channels determines the
average conductance. In the case of general incommensurate tubes, separation of
degenerated energy levels due to intertube transfer is suppressed in the energy
region higher than the Fermi energy but not in the energy region lower than
that. Accordingly, in the former case, there are few effects of intertube
transfer on the conductance, while in the latter case, separation of
degenerated energy levels leads to large reduction of the conductance. It is
also found that in some cases antiresonance with edge states in inner tubes
causes an anomalous conductance quantization, , near the Fermi
energy.Comment: 24 pages, 13 figures, to be published in Physical Review
Vitamin E reduces amyloidosis and improves cognitive function in Tg2576 mice following repetitive concussive brain injury
Traumatic brain injury is a well-recognized environmental risk factor for developing Alzheimer's disease. Repetitive concussive brain injury (RCBI) exacerbates brain lipid peroxidation, accelerates amyloid (Abeta) formation and deposition, as well as cognitive impairments in Tg2576 mice. This study evaluated the effects of vitamin E on these four parameters in Tg2576 mice following RCBI. Eleven-month-old mice were randomized to receive either regular chow or chow-supplemented with vitamin E for 4 weeks, and subjected to RCBI (two injuries, 24 h apart) using a modified controlled cortical impact model of closed head injury. The same dietary regimens were maintained up to 8 weeks post-injury, when the animals were killed for biochemical and immunohistochemical analyses after behavioral evaluation. Vitamin E-treated animals showed a significant increase in brain vitamin E levels and a significant decrease in brain lipid peroxidation levels. After RBCI, compared with the group on regular chow, animals receiving vitamin E did not show the increase in Abeta peptides, and had a significant attenuation of learning deficits. This study suggests that the exacerbation of brain oxidative stress following RCBI plays a mechanistic role in accelerating Abeta accumulation and behavioral impairments in the Tg2576 mice
Optical response of finite-length carbon nanotubes
Optical response of finite-length metallic carbon nanotubes is calculated
including effects of induced edge charges in a self-consistent manner. The
results show that the main resonance corresponding to excitation of the
fundamental plasmon mode with wave vector with being the tube
length is quite robust and unaffected. This arises because the strong electric
field associated with edge charges is screened and decays rapidly inside the
nanotube. For higher-frequency resonances, the field starts to be mixed and
tends to shift resonances to higher frequencies.Comment: 10 pages, 9 figures, to be published in J. Phys. Soc. Jp
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