205 research outputs found
A Nonlinear Coupling Network to Simulate the Development of the r-mode Instablility in Neutron Stars II. Dynamics
Two mechanisms for nonlinear mode saturation of the r-mode in neutron stars
have been suggested: the parametric instability mechanism involving a small
number of modes and the formation of a nearly continuous Kolmogorov-type
cascade. Using a network of oscillators constructed from the eigenmodes of a
perfect fluid incompressible star, we investigate the transition between the
two regimes numerically. Our network includes the 4995 inertial modes up to n<=
30 with 146,998 direct couplings to the r-mode and 1,306,999 couplings with
detuning< 0.002 (out of a total of approximately 10^9 possible couplings).
The lowest parametric instability thresholds for a range of temperatures are
calculated and it is found that the r-mode becomes unstable to modes with
13<n<15. In the undriven, undamped, Hamiltonian version of the network the rate
to achieve equipartition is found to be amplitude dependent, reminiscent of the
Fermi-Pasta-Ulam problem. More realistic models driven unstable by
gravitational radiation and damped by shear viscosity are explored next. A
range of damping rates, corresponding to temperatures 10^6K to 10^9K, is
considered. Exponential growth of the r-mode is found to cease at small
amplitudes, approximately 10^-4. For strongly damped, low temperature models, a
few modes dominate the dynamics. The behavior of the r-mode is complicated, but
its amplitude is still no larger than about 10^-4 on average. For high
temperature, weakly damped models the r-mode feeds energy into a sea of
oscillators that achieve approximate equipartition. In this case the r-mode
amplitude settles to a value for which the rate to achieve equipartition is
approximately the linear instability growth rate.Comment: 18 Pages 14 Figure
A Pulsational Model for the Orthogonal Polarization Modes in Radio Pulsars
In an earlier paper, we introduced a model for pulsars in which non-radial
oscillations of high spherical degree (\el) aligned to the magnetic axis of a
spinning neutron star were able to reproduce subpulses like those observed in
single-pulse measurements of pulsar intensity. The model did not address
polarization, which is an integral part of pulsar emission. Observations show
that many pulsars emit radio waves that appear to be the superposition of two
linearly polarized emission modes with orthogonal polarization angles. In this
paper, we extend our model to incorporate linear polarization. As before, we
propose that pulsational displacements of stellar material modulate the pulsar
emission, but now we apply this modulation to a linearly-polarized mode of
emission, as might be produced by curvature radiation. We further introduce a
second polarization mode, orthogonal to the first, that is modulated by
pulsational velocities. We combine these modes in superposition to model the
observed Stokes parameters in radio pulsars.Comment: 19 pages, 4 figures accepted Ap
Front Form Spinors in Weinberg-Soper Formalism and Melosh Transformations for any Spin
Using the Weinberg-Soper formalism we construct the front form
spinors. Explicit expressions for the generalised Melosh
transformations up to spin two are obtained. The formalism, without explicitly
invoking any wave equations, reproduces spin one half front-form results of
Melosh, Lepage and Brodsky, and Dziembowski.Comment: 16 Pages, RevTex. We continue to receive reprint requests for this
paper. So we now archive it her
Helioseismology, solar models and neutrino fluxes
We present our results concerning a systematical analysis of helioseismic
implications on solar structure and neutrino production. We find
Y, and
gr/cm. In the interval , the quantity is
determined with and accuracy of \permille~or better. At the solar center
still one has remarkable accuracy, . We compare the predictions
of recent solar models (standard and non-standard) with the helioseismic
results. By constructing helioseismically constrained solar models, the central
solar temperature is found to be K with a conservatively
estimated accuracy of 1.4%, so that the major unceratainty on neutrino fluxes
is due to nuclear cross section and not to solar inputs.Comment: 14 pages including 9 figures, LaTex file, espcrc2.sty is needed; to
appear in Nucl. Phys. B Proc. Suppl., Proceedings of TAUP97 conference,
Laboratori Nazionali del Gran Sasso, September 199
Quantitative proteomics revealed C6orf203/MTRES1 as a factor preventing stress-induced transcription deficiency in human mitochondria
Maintenance of mitochondrial gene expression is
crucial for cellular homeostasis. Stress conditions
may lead to a temporary reduction of mitochondrial
genome copy number, raising the risk of insufficient
expression of mitochondrial encoded genes. Little
is known how compensatory mechanisms operate
to maintain proper mitochondrial transcripts levels
upon disturbed transcription and which proteins are
involved in them. Here we performed a quantitative
proteomic screen to search for proteins that sustain
expression of mtDNA under stress conditions. Analysis
of stress-induced changes of the human mitochondrial
proteome led to the identification of several
proteins with poorly defined functions among which
we focused on C6orf203, which we named MTRES1
(Mitochondrial Transcription Rescue Factor 1). We
found that the level of MTRES1 is elevated in cells
under stress and we show that this upregulation of
MTRES1 prevents mitochondrial transcript loss under
perturbed mitochondrial gene expression. This
protective effect depends on the RNA binding activity
of MTRES1. Functional analysis revealed that
MTRES1 associates with mitochondrial RNA polymerase
POLRMT and acts by increasing mitochondrial
transcription, without changing the stability of
mitochondrial RNAs. We propose that MTRES1 is an
example of a protein that protects the cell from mitochondrial
RNA loss during stress
Nucleon electroweak form factors in a meson-cloud model
The meson-cloud model of the nucleon consisting of a system of three valence
quarks surrounded by a meson cloud is applied to study the electroweak
structure of the proton and neutron. The electroweak nucleon form factors are
calculated within a light-front approach, by obtaining an overall good
description of the experimental data. Charge densities as a function of the
transverse distance with respect to the direction of the three-momentum
transfer are also discussed.Comment: Prepared for Proceedings of NSTAR2007, Workshop on the physics of
excited nucleons, Bonn (Germany), 5-8 September 200
Rotational covariance and light-front current matrix elements
Light-front current matrix elements for elastic scattering from hadrons with
spin~1 or greater must satisfy a nontrivial constraint associated with the
requirement of rotational covariance for the current operator. Using a model
meson as a prototype for hadronic quark models, this constraint and its
implications are studied at both low and high momentum transfers. In the
kinematic region appropriate for asymptotic QCD, helicity rules, together with
the rotational covariance condition, yield an additional relation between the
light-front current matrix elements.Comment: 16 pages, [no number
Helioseismic constraints to the central solar temperature and neutrino fluxes
The central solar temperature T and its uncertainties are calculated in
helioseismologically-constrained solar models. From the best fit to the
convective radius, density at the convective radius and seismically determined
helium abundance the central temperature is found to be T=1.58x10^7 K, in
excellent agreement with Standard Solar Models. Conservatively, we estimate
that the accuracy of this determination is Delta T/T=1.4 %, better than that in
SSM. Neutrino fluxes are calculated. The lower limit to the boron neutrino
flux, obtained with maximum reduction factors from all sources of
uncertainties, is 2 sigma higher than the flux measured recently by
SuperKamiokande.Comment: Latex, 10 pages, uses revtex styles (included), 3 postscript figure
Baryon Current Matrix Elements in a Light-Front Framework
Current matrix elements and observables for electro- and photo-excitation of
baryons from the nucleon are studied in a light-front framework. Relativistic
effects are estimated by comparison to a nonrelativistic model, where we use
simple basis states to represent the baryon wavefunctions. Sizeable
relativistic effects are found for certain transitions, for example, to radial
excitations such as that conventionally used to describe to the Roper
resonance. A systematic study shows that the violation of rotational covariance
of the baryon transition matrix elements stemming from the use of one-body
currents is generally small.Comment: 32 pages, LaTeX, 10 postscript figures, uses epsf.sty; figures
uuencoded with uufiles (or available by request in .ps or hardcopy form
Comments on Exclusive Electroproduction of Transversely Polarized Vector Mesons
We discuss the electroproduction of light vector mesons from transversely
polarized photons. Here QCD factorization cannot be applied as shown explicitly
in a leading order calculation of corresponding Feynman diagrams. It is
emphasized that present infrared singular contributions cannot be regularized
through phenomenological meson distribution amplitudes with suppressed endpoint
configurations. We point out that infrared divergencies arise also from
integrals over skewed parton distributions of the nucleons.
In a phenomenological analysis of transverse vector meson production model
dependent regularizations have to be applied. If this procedure preserves the
analytic structure suggested by a leading order calculation of Feynman
diagrams, one obtains contributions from nucleon parton distributions and their
derivatives. In particular polarized gluons enter only through their
derivative
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