178 research outputs found
Radiation pattern of the isolated pulsar PSR B1828-11
Based on the free precession model of the isolated pulsar PSR B1828-11, Link
& Epstein 2001) showed that the observed pulse durations require the radio beam
to have a non-standard shape: the beam duration is larger for beam sweeps
farthest from the dipole axis. In their analysis they assumed that the actual
precession period is ~ 500 d. Recent theoretical studies suggested that the
actual precession period might be ~ 1000 d as seen in observations (Rezania
2002, Wasserman 2002). In this paper, in a good agreement with the observed
data (Stairs et al. 2000), we model the changes of the pulse shape in a
precession cycle with period ~ 1000 d and find that the variation of the pulse
duration follows from a {\it standard} beam pattern in each cycle.Comment: 9 pages, 3 figures, Accepted for publication in A&
Vorticity affects the stability of neutron stars
The spin rate Omega of neutron stars at a given temperature T is constrained by the interplay between gravitational-radiation instabilities and viscous damping. Navier-Stokes theory has been used to calculate the viscous damping timescales and produce a stability curve for r-modes in the (Omega,T) plane. In Navier-Stokes theory, viscosity is independent of vorticity, but kinetic theory predicts a coupling of vorticity to the shear viscosity. We calculate this coupling and show that it can in principle significantly modify the stability diagram at lower temperatures. As a result, colder stars can remain stable at higher spin rates
On the precession of the isolated pulsar PSR B1828-11
Analysis of both pulse timing and pulse shape variations of the isolated
pulsar PSR B1828-11 shows highly correlated and strong Fourier power at periods
\~ 1000, 500, and 250 d (Stairs et al. 2000). The only description based on a
free precession of star's rigid crust coupled to the magnetic dipole torque,
explains the 500-component, as the fundamental Fourier frequency, with its
harmonic 250-component (Link & Epstein 2001). In this paper, we show that if
the dipole moment vector varies with time with a period nearly equal to the
longest (probably fundamental) observed period (~ 1000 d), the dipole torque
may produce the all other harmonics. We also find the second and fourth
harmonics at periods ~ 500 and 250 d are dominant for small wobble angle ~ 3^o
and large field's inclination angle 89^o.Comment: 11 pages, discussion is change
Model of ionic currents through microtubule nanopores and the lumen
It has been suggested that microtubules and other cytoskeletal filaments may
act as electrical transmission lines. An electrical circuit model of the
microtubule is constructed incorporating features of its cylindrical structure
with nanopores in its walls. This model is used to study how ionic conductance
along the lumen is affected by flux through the nanopores when an external
potential is applied across its two ends. Based on the results of Brownian
dynamics simulations, the nanopores were found to have asymmetric inner and
outer conductances, manifested as nonlinear IV curves. Our simulations indicate
that a combination of this asymmetry and an internal voltage source arising
from the motion of the C-terminal tails causes a net current to be pumped
across the microtubule wall and propagate down the microtubule through the
lumen. This effect is demonstrated to enhance and add directly to the
longitudinal current through the lumen resulting from an external voltage
source, and could be significant in amplifying low-intensity endogenous
currents within the cellular environment or as a nano-bioelectronic device.Comment: 43 pages, 6 figures, revised versio
Kinetic Theory of Collisionless Self-Gravitating Gases: Post-Newtonian Polytropes
In this paper we study the kinetic theory of many-particle astrophysical
systems and we present a consistent version of the collisionless Boltzmann
equation in the 1PN approximation. We argue that the equation presented by
Rezania and Sobouti in A&A 354 1110 (2000) is not the correct expression to
describe the evolution of a collisionless self-gravitating gas. One of the
reasons that account for the previous statement is that the energy of a
free-falling test particle, obeying the 1PN equations of motion for static
gravitational fields, is not a static solution of the mentioned equation. The
same statement holds for the angular momentum, in the case of spherical
systems. We provide the necessary corrections and obtain an equation that is
consistent with the corresponding equations of motion and the 1PN conserved
quantities. We suggest some potential relevance for the study of high density
astrophysical systems and as an application we construct the corrected version
of the post-Newtonian polytropes.Comment: 23 pages, 24 figures. Accepted for publication in PR
Green's function approach to quantum criticality in the anisotropic Kondo-necklace model
We have studied the quantum phase transition between the antiferromagnetic
and spin liquid phase for the two dimensional anisotropic Kondo-necklace model.
The bond operator formalism has been implemented to transform the spin
Hamiltonian to a bosonic one. We have used the Green's function approach
including a hard core repulsion to find the low energy excitation spectrum of
the model. The bosonic excitations become gapless at the quantum critical point
where the phase transition from the Kondo singlet state to long range
antiferromagnetic order takes place. We have studied the effect of both
inter-site (delta) and local (Delta) anisotropies on the critical point and on
the critical exponent of the excitation gap in the paramagnetic phase. We have
also compared our results with previous bond operator mean field calculations.Comment: 14 pages and 7 EPS figure
The r-modes of rotating fluids
An analysis of the toroidal modes of a rotating fluid, by means of the
differential equations of motion is not readily tractable. A matrix
representation of the equations in a suitable basis, however, simplifies the
problem considerably and reveals many of its intricacies.Comment: 12 pages, 4 fiures, revised version to appear in A&
Large frequency drifts during Type I X-ray bursts
We study the spin-down of a neutron star atmosphere during the Type I X-ray
burst in low mass X-ray binaries. Using polar cap acceleration models, we show
that the resulting stellar ``wind'' torque on the burning shell due to the
flowing charged particles (electrons, protons and ions) from the star's polar
caps may change the shell's angular momentum during the burst. We conclude that
the net change in the angular momentum of the star's atmosphere can account for
rather large frequency drifts observed during Type I X-ray burst.Comment: 8 pages, more discussion adde
RGDS peptides immobilized on titanium alloy stimulate bone cell attachment, differentiation and confer resistance to apoptosis
A major cause of implant failure in skeletal tissues is failure of osseointegration, often due to lack of adhesion of cells to the titanium (Ti) alloy interface. Since arginine- glycine-aspartic acid (RGD)-containing peptides have been shown to regulate osteoblast adhesion, we tested the hypothesis that, bound to a Ti surface, these peptides would promote osteoblasts differentiation, while at the same time inhibit apoptosis. RGDS and RGES (control) peptides were covalently linked to Ti discs using an APTS linker. While the grafting of both RGDS and RGES significantly increased Ti surface roughness, contact angle analysis showed that APTS significantly increased the surface hydrophobicity; when the peptides were tethered to Ti, this was reduced. To evaluate attachment, MC3T3-E1 osteoblast cells were grown on these discs. Significantly more cells attached to the Ti-grafted RGDS then the Ti-grafted RGES control. Furthermore, expression of the osteoblasts phenotype was significantly enhanced on the Ti-grafted RGDS surface. When cells attached to the Ti-grafted RGDS were challenged with staurosporine, an apoptogen, there was significant inhibition of apoptosis; in contrast, osteoblasts adherent to the Ti-grafted RGES were killed. It is concluded that RGD-containing peptides covalently bonded to Ti promotes osteoblasts attachment and survival with minimal changes to the surface of the alloy. Therefore, such modifications to Ti would have the potential to promote osseointegration in vivo
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