276 research outputs found
Spin-up of the hyperon-softened accreting neutron stars
We study the spin-up of the accreting neutron stars with a realistic
hyperon-softened equation of state. Using precise 2-D calculations we study the
evolutionary tracks of accreting neutron stars in the angular-momentum -
frequency plane. In contrast to the case of spinning-down solitary
radio-pulsars, where a strong back-bending behavior has been observed, we do
not see back-bending phenomenon in the accretion-powered spinning-up case. We
conclude that in the case of accretion-driven spin-up the back-bending is
strongly suppressed by the mass-increase effect accompanying the
angular-momentum increase.Comment: 5 pages, 5 figures, accepted by Astronomy & Astrophysic
Tunneling and Non-Universality in Continuum Percolation Systems
The values obtained experimentally for the conductivity critical exponent in
numerous percolation systems, in which the interparticle conduction is by
tunnelling, were found to be in the range of and about , where
is the universal conductivity exponent. These latter values are however
considerably smaller than those predicted by the available ``one
dimensional"-like theory of tunneling-percolation. In this letter we show that
this long-standing discrepancy can be resolved by considering the more
realistic "three dimensional" model and the limited proximity to the
percolation threshold in all the many available experimental studiesComment: 4 pages, 2 figure
Theory of continuum percolation I. General formalism
The theoretical basis of continuum percolation has changed greatly since its
beginning as little more than an analogy with lattice systems. Nevertheless,
there is yet no comprehensive theory of this field. A basis for such a theory
is provided here with the introduction of the Potts fluid, a system of
interacting -state spins which are free to move in the continuum. In the limit, the Potts magnetization, susceptibility and correlation functions
are directly related to the percolation probability, the mean cluster size and
the pair-connectedness, respectively. Through the Hamiltonian formulation of
the Potts fluid, the standard methods of statistical mechanics can therefore be
used in the continuum percolation problem.Comment: 26 pages, Late
Surface gravity of neutron stars and strange stars
The upper bound on the value of the surface gravity, g_s, for neutron stars
with equations of state respecting v_sound <= c, is derived. This bound is
inversely proportional to the maximum allowable mass M_max, and it reads g_s <=
1.411 x 10^15 (M_sun/M_max) cm/s^2. It implies an absolute upper bound 7.4 x
10^14 cm/s^2 if one uses the 2sigma lower bound on the neutron mass measured
recently in 4U1700-37, 1.9 M_sun. A correlation between g_s and the compactness
parameter 2GM/Rc^2 for baryonic stars is analyzed. The properties of g_s of
strange quark stars and its upper bounds are discussed using the scaling
properties of the strange-star models.Comment: Accepted by A&
Keplerian frequency of uniformly rotating neutron stars and quark stars
We calculate Keplerian (mass shedding) configurations of rigidly rotating
neutron stars and quark stars with crusts. We check the validity of empirical
formula for Keplerian frequency, f_K, proposed by Lattimer & Prakash, f_K(M)=C
(M/M_sun)^1/2 (R/10km)^-3/2, where M is the (gravitational) mass of Keplerian
configuration, R is the (circumferential) radius of the non-rotating
configuration of the same gravitational mass, and C = 1.04 kHz. Numerical
calculations are performed using precise 2-D codes based on the multi-domain
spectral methods. We use a representative set of equations of state (EOSs) of
neutron stars and quark stars. We show that the empirical formula for f_K(M)
holds within a few percent for neutron stars with realistic EOSs, provided 0.5
M_sun < M < 0.9 M_max,stat, where M_max,stat is the maximum allowable mass of
non-rotating neutron stars for an EOS, and C=C_NS=1.08 kHz. Similar precision
is obtained for quark stars with 0.5 M_sun < M < 0.9 M_max,stat. For maximal
crust masses we obtain C_QS = 1.15 kHz, and the value of C_QS is not very
sensitive to the crust mass. All our C's are significantly larger than the
analytic value from the relativistic Roche model, C_Roche = 1.00 kHz. For 0.5
M_sun < M < 0.9 M_max,stat, the equatorial radius of Keplerian configuration of
mass M, R_K(M), is, to a very good approximation, proportional to the radius of
the non-rotating star of the same mass, R_K(M) = aR(M), with a_NS \approx a_QS
\approx 1.44. The value of a_QS is very weakly dependent on the mass of the
crust of the quark star. Both a's are smaller than the analytic value a_Roche =
1.5 from the relativistic Roche model.Comment: 6 pages, 6 color figures, submitted to A&
Exact solution of a one-dimensional continuum percolation model
I consider a one dimensional system of particles which interact through a
hard core of diameter \si and can connect to each other if they are closer
than a distance . The mean cluster size increases as a function of the
density until it diverges at some critical density, the percolation
threshold. This system can be mapped onto an off-lattice generalization of the
Potts model which I have called the Potts fluid, and in this way, the mean
cluster size, pair connectedness and percolation probability can be calculated
exactly. The mean cluster size is S = 2 \exp[ \rho (d -\si)/(1 - \rho \si)] -
1 and diverges only at the close packing density \rho_{cp} = 1 / \si . This
is confirmed by the behavior of the percolation probability. These results
should help in judging the effectiveness of approximations or simulation
methods before they are applied to higher dimensions.Comment: 21 pages, Late
Percolation-to-hopping crossover in conductor-insulator composites
Here, we show that the conductivity of conductor-insulator composites in
which electrons can tunnel from each conducting particle to all others may
display both percolation and tunneling (i.e. hopping) regimes depending on few
characteristics of the composite. Specifically, we find that the relevant
parameters that give rise to one regime or the other are (where is
the size of the conducting particles and is the tunneling length) and the
specific composite microstructure. For large values of , percolation
arises when the composite microstructure can be modeled as a regular lattice
that is fractionally occupied by conducting particle, while the tunneling
regime is always obtained for equilibrium distributions of conducting particles
in a continuum insulating matrix. As decreases the percolating behavior
of the conductivity of lattice-like composites gradually crosses over to the
tunneling-like regime characterizing particle dispersions in the continuum. For
values lower than the conductivity has tunneling-like
behavior independent of the specific microstructure of the composite.Comment: 8 pages, 5 figure
S-wave Pairing of Hyperons in Dense Matter
In this work we calculate the gap energies of hyperons in
neutron star matter. The calculation is based on a solution of the BCS gap
equation for an effective G-matrix parameterization of the
interaction with a nuclear matter background, presented recently by Lanskoy and
Yamamoto. We find that a gap energy of a few tenths of MeV is expected for
Fermi momenta up to about 1.3 fm. Implications for neutron
star matter are examined, and suggest the existence of a
superfluid between the threshold baryon density for formation and the
baryon density where the fraction reaches .Comment: 16 pages, Revtex, 9 figures, 33 reference
Hyperon-hyperon interactions and properties of neutron matter
We present results from Brueckner-Hartree-Fock calculatons for beta stable
neutron star matter with nucleonic and hyperonic degress degrees of freedom,
employing the most recent parametrizations of the baryon-baryon interaction of
the Nijmegen group. It is found that the only strange baryons emergin in beta
stable matter up to total barionic densities of 1.2 fm^-3 are and
. The corresponding equations of state are then used to compute
properties of neutron stars such as masses and radii.Comment: 27 pages, LateX, includes 8 PostScript figures, (submitted to PRC
Solution of the tunneling-percolation problem in the nanocomposite regime
We noted that the tunneling-percolation framework is quite well understood at
the extreme cases of percolation-like and hopping-like behaviors but that the
intermediate regime has not been previously discussed, in spite of its
relevance to the intensively studied electrical properties of nanocomposites.
Following that we study here the conductivity of dispersions of particle
fillers inside an insulating matrix by taking into account explicitly the
filler particle shapes and the inter-particle electron tunneling process. We
show that the main features of the filler dependencies of the nanocomposite
conductivity can be reproduced without introducing any \textit{a priori}
imposed cut-off in the inter-particle conductances, as usually done in the
percolation-like interpretation of these systems. Furthermore, we demonstrate
that our numerical results are fully reproduced by the critical path method,
which is generalized here in order to include the particle filler shapes. By
exploiting this method, we provide simple analytical formulas for the composite
conductivity valid for many regimes of interest. The validity of our
formulation is assessed by reinterpreting existing experimental results on
nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the
characteristic tunneling decay length, which is found to be within the expected
range of its values. These results are concluded then to be not only useful for
the understanding of the intermediate regime but also for tailoring the
electrical properties of nanocomposites.Comment: 13 pages with 8 figures + 10 pages with 9 figures of supplementary
material (Appendix B
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