9,941 research outputs found
Compelled to do the right thing
We use a model of opinion formation to study the consequences of some
mechanisms attempting to enforce the right behaviour in a society. We start
from a model where the possible choices are not equivalent (such is the case
when the agents decide to comply or not with a law) and where an imitation
mechanism allow the agents to change their behaviour based on the influence of
a group of partners. In addition, we consider the existence of two social
constraints: a) an external authority, called monitor, that imposes the correct
behaviour with infinite persuasion and b) an educated group of agents that act
upon their fellows but never change their own opinion, i.e., they exhibit
infinite adamancy. We determine the minimum number of monitors to induce an
effective change in the behaviour of the social group, and the size of the
educated group that produces the same effect. Also, we compare the results for
the cases of random social interactions and agents placed on a network. We have
verified that a small number of monitors are enough to change the behaviour of
the society. This also happens with a relatively small educated group in the
case of random interactions.Comment: 8 pages, 9 figures, submitted to EPJ
Modeling Two Dimensional Magnetic Domain Patterns
Two-dimensional magnetic garnets exhibit complex and fascinating magnetic
domain structures, like stripes, labyrinths, cells and mixed states of stripes
and cells. These patterns do change in a reversible way when the intensity of
an externally applied magnetic field is varied. The main objective of this
contribution is to present the results of a model that yields a rich pattern
structure that closely resembles what is observed experimentally. Our model is
a generalized two-dimensional Ising-like spin-one Hamiltonian with long-range
interactions, which also incorporates anisotropy and Zeeman terms. The model is
studied numerically, by means of Monte Carlo simulations. Changing the model
parameters stripes, labyrinth and/or cellular domain structures are generated.
For a variety of cases we display the patterns, determine the average size of
the domains, the ordering transition temperature, specific heat, magnetic
susceptibility and hysteresis cycle. Finally, we examine the reversibility of
the pattern evolution under variations of the applied magnetic field. The
results we obtain are in good qualitative agreement with experiment.Comment: 8 pages, 12 figures, submitted to Phys. Rev.
A refined analysis of the low-mass eclipsing binary system T-Cyg1-12664
The observational mass-radius relation of main sequence stars with masses
between ~0.3 and 1.0 Msun reveals deviations between the stellar radii
predicted by models and the observed radii of stars in detached binaries. We
generate an accurate physical model of the low-mass eclipsing binary
T-Cyg1-12664 in the Kepler mission field to measure the physical parameters of
its components and to compare them with the prediction of theoretical stellar
evolution models. We analyze the Kepler mission light curve of T-Cyg1-12664 to
accurately measure the times and phases of the primary and secondary eclipse.
In addition, we measure the rotational period of the primary component by
analyzing the out-of-eclipse oscillations that are due to spots. We accurately
constrain the effective temperature of the system using ground-based absolute
photometry in B, V, Rc, and Ic. We also obtain and analyze V, Rc, Ic
differential light curves to measure the eccentricity and the orbital
inclination of the system, and a precise Teff ratio. From the joint analysis of
new radial velocities and those in the literature we measure the individual
masses of the stars. Finally, we use the PHOEBE code to generate a physical
model of the system. T-Cyg1-12664 is a low eccentricity system, located
d=360+/-22 pc away from us, with an orbital period of P=4.1287955(4) days, and
an orbital inclination i=86.969+/-0.056 degrees. It is composed of two very
different stars with an active G6 primary with Teff1=5560+/-160 K,
M1=0.680+/-0.045 Msun, R1=0.799+/-0.017 Rsun, and a M3V secondary star with
Teff2=3460+/-210 K, M2=0.376+/-0.017 Msun, and R2=0.3475+/-0.0081 Rsun. The
primary star is an oversized and spotted active star, hotter than the stars in
its mass range. The secondary is a cool star near the mass boundary for fully
convective stars (M~0.35 Msun), whose parameters appear to be in agreement with
low-mass stellar model.Comment: 18 pages, 15 figures, 15 table
Model Calculation of Electron-Phonon Couplings in a Dimer with a Non-Degenerate Orbital
We evaluate all the electron-phonon couplings derived from the one-body
electronic interactions, in both the adiabatic and extreme non-adiabatic limit,
for a dimer with a non-degenerate orbital built from atomic wave functions of
Gaussian shape. We find largely different values of the coupling parameters in
the two cases, as well as different expressions of the corresponding terms in
the Hamiltonian.Comment: 5 postscript figure
Entropy and equilibrium state of free market models
Many recent models of trade dynamics use the simple idea of wealth exchanges
among economic agents in order to obtain a stable or equilibrium distribution
of wealth among the agents. In particular, a plain analogy compares the wealth
in a society with the energy in a physical system, and the trade between agents
to the energy exchange between molecules during collisions. In physical
systems, the energy exchange among molecules leads to a state of equipartition
of the energy and to an equilibrium situation where the entropy is a maximum.
On the other hand, in the majority of exchange models, the system converges to
a very unequal condensed state, where one or a few agents concentrate all the
wealth of the society while the wide majority of agents shares zero or almost
zero fraction of the wealth. So, in those economic systems a minimum entropy
state is attained. We propose here an analytical model where we investigate the
effects of a particular class of economic exchanges that minimize the entropy.
By solving the model we discuss the conditions that can drive the system to a
state of minimum entropy, as well as the mechanisms to recover a kind of
equipartition of wealth
Electromagnetic emission of white dwarf binary mergers
It has been recently proposed that the ejected matter from white dwarf (WD)
binary mergers can produce transient, optical and infrared emission similar to
the "kilonovae" of neutron star (NS) binary mergers. To confirm this we
calculate the electromagnetic emission from WD-WD mergers and compare with
kilonova observations. We simulate WD-WD mergers leading to a massive, fast
rotating, highly magnetized WD with an adapted version of the
smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial
conditions for the ejecta such as escape velocity, mass and initial position
and distribution. The subsequent thermal and dynamical evolution of the ejecta
is obtained by integrating the energy-conservation equation accounting for
expansion cooling and a heating source given by the fallback accretion onto the
newly-formed WD and its magneto-dipole radiation. We show that magnetospheric
processes in the merger can lead to a prompt, short gamma-ray emission of up to
erg in a timescale of - s. The bulk of the ejecta
initially expands non-relativistically with velocity and then it
accelerates to due to the injection of fallback accretion energy. The
ejecta become transparent at optical wavelengths around days
post-merger with a luminosity - erg s. The X-ray
emission from the fallback accretion becomes visible around -
day post-merger with a luminosity of erg s. We also predict
the post-merger time at which the central WD should appear as a pulsar
depending on the value of the magnetic field and rotation period.Comment: 12 pages, Accepted for publication in JCA
Magnetic Field scaling of Relaxation curves in Small Particle Systems
We study the effects of the magnetic field on the relaxation of the
magnetization of small monodomain non-interacting particles with random
orientations and distribution of anisotropy constants. Starting from a master
equation, we build up an expression for the time dependence of the
magnetization which takes into account thermal activation only over barriers
separating energy minima, which, in our model, can be computed exactly from
analytical expressions. Numerical calculations of the relaxation curves for
different distribution widths, and under different magnetic fields H and
temperatures T, have been performed. We show how a \svar scaling of the
curves, at different T and for a given H, can be carried out after proper
normalization of the data to the equilibrium magnetization. The resulting
master curves are shown to be closely related to what we call effective energy
barrier distributions, which, in our model, can be computed exactly from
analytical expressions. The concept of effective distribution serves us as a
basis for finding a scaling variable to scale relaxation curves at different H
and a given T, thus showing that the field dependence of energy barriers can be
also extracted from relaxation measurements.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
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