12,007 research outputs found
Neutron Stars with Bose-Einstein Condensation of Antikaons as MIT Bags
We investigate the properties of an antikaon in medium, regarding itas a MIT
bag. We first construct the MIT bag model for a kaon with and
in order to describe the interaction of-quarks in hyperonic matter in the
framework of the modifiedquark-meson coupling model. The coupling constant
in the density-dependent bag constant is treated
as afree parameter to reproduce the optical potential of a kaon in asymmetric
matter and all other couplings are determined by usingSU(6) symmetry and the
quark counting rule. With various values ofthe kaon potential, we calculate the
effective mass of a kaon inmedium to compare it with that of a point-like kaon.
We thencalculate the population of octet baryons, leptons and and
theequation of state for neutron star matter. The results show thatkaon
condensation in hyperonic matter is sensitive to the -quarkinteraction and
also to the way of treating the kaon. The mass andthe radius of a neutron star
are obtained by solving theTolmann-Oppenheimer-Volkoff equation.Comment: 14 figure
Kaon Condensation in a Neutron Star under Strong Magnetic Fields by Using the Modified Quark-meson Coupling Model
We have considered the antikaon condensation in a neutron star in the
presence of strong magnetic fields by using the modified quark-meson coupling
(MQMC) model. The structure of the neutron star is investigated with various
magnetic fields and different kaon optical potentials, and the effects of the
magnetic fields for kaon condensation is discussed. When employing strong
magnetic fields inside a neutron star with hyperons and kaon condensation, the
magnetic fields can cause the equation of state to be stiff; thus, a large
maximum mass of the neutron star can be obtained.Comment: published in J. Korean Phys.So
Thermotaxis in Caenorhabditis elegans analyzed by measuring responses to defined thermal stimuli
In a spatial thermal gradient, Caenorhabditis elegans migrates toward and then isothermally tracks near its cultivation temperature. A current model for thermotactic behavior involves a thermophilic drive (involving the neurons AFD and AIY) and cryophilic drive (involving the neuron AIZ) that balance at the cultivation temperature. Here, we analyze the movements of individual worms responding to defined thermal gradients. We found evidence for a mechanism for migration down thermal gradients that is active at temperatures above the cultivation temperature, and a mechanism for isothermal tracking that is active near the cultivation temperature. However, we found no evidence for a mechanism for migration up thermal gradients at temperatures below the cultivation temperature that might have supported the model of opposing drives. The mechanisms for migration down gradients and isothermal tracking control the worm's movements in different manners. Migration down gradients works by shortening (lengthening) the duration of forward movement in response to positive (negative) temperature changes. Isothermal tracking works by orienting persistent forward movement to offset temperature changes. We believe preference for the cultivation temperature is not at the balance between two drives. Instead, the worm activates the mechanism for isothermal tracking near the cultivation temperature and inactivates the mechanism for migration down gradients near or below the cultivation temperature. Inactivation of the mechanism for migration down gradients near or below the cultivation temperature requires the neurons AFD and AIY
Three-dimensional Magnetohydrodynamic Simulations of Buoyant Bubbles in Galaxy Clusters
We report results of 3D MHD simulations of the dynamics of buoyant bubbles in
magnetized galaxy cluster media. The simulations are three dimensional
extensions of two dimensional calculations reported by Jones & De Young (2005).
Initially spherical bubbles and briefly inflated spherical bubbles all with
radii a few times smaller than the intracluster medium (ICM) scale height were
followed as they rose through several ICM scale heights. Such bubbles quickly
evolve into a toroidal form that, in the absence of magnetic influences, is
stable against fragmentation in our simulations. This ring formation results
from (commonly used) initial conditions that cause ICM material below the
bubbles to drive upwards through the bubble, creating a vortex ring; that is,
hydrostatic bubbles develop into "smoke rings", if they are initially not very
much smaller or very much larger than the ICM scale height. Even modest ICM
magnetic fields with beta = P_gas/P_mag ~ 10^3 can influence the dynamics of
the bubbles, provided the fields are not tangled on scales comparable to or
smaller than the size of the bubbles. Quasi-uniform, horizontal fields with
initial beta ~ 10^2 bifurcated our bubbles before they rose more than about a
scale height of the ICM, and substantially weaker fields produced clear
distortions. On the other hand, tangled magnetic fields with similar, modest
strengths are generally less easily amplified by the bubble motions and are
thus less influential in bubble evolution. Inclusion of a comparably strong,
tangled magnetic field inside the initial bubbles had little effect on our
bubble evolution, since those fields were quickly diminished through expansion
of the bubble and reconnection of the initial field.Comment: 20 pages, 12 figures. Accepted for publication in The Astrophysical
Journa
Interaction effects on 2D fermions with random hopping
We study the effects of generic short-ranged interactions on a system of 2D
Dirac fermions subject to a special kind of static disorder, often referred to
as ``chiral.'' The non-interacting system is a member of the disorder class BDI
[M. R. Zirnbauer, J. Math. Phys. 37, 4986 (1996)]. It emerges, for example, as
a low-energy description of a time-reversal invariant tight-binding model of
spinless fermions on a honeycomb lattice, subject to random hopping, and
possessing particle-hole symmetry. It is known that, in the absence of
interactions, this disordered system is special in that it does not localize in
2D, but possesses extended states and a finite conductivity at zero energy, as
well as a strongly divergent low-energy density of states. In the context of
the hopping model, the short-range interactions that we consider are
particle-hole symmetric density-density interactions. Using a perturbative
one-loop renormalization group analysis, we show that the same mechanism
responsible for the divergence of the density of states in the non-interacting
system leads to an instability, in which the interactions are driven strongly
relevant by the disorder. This result should be contrasted with the limit of
clean Dirac fermions in 2D, which is stable against the inclusion of weak
short-ranged interactions. Our work suggests a novel mechanism wherein a clean
system, initially insensitive to interaction effects, can be made unstable to
interactions upon the inclusion of weak static disorder.Comment: 16 pages, 10 figures; References added, figures enlarged; to be
published in Phys. Rev.
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