1,379 research outputs found
Self-Organized Bottleneck in Energy Relaxation
We study an energy relaxation process after many degrees of freedom are
excited in a Hamiltonian system with a large number of degrees of freedom.
Bottlenecks of relaxation, where relaxations of the excited elements are
drastically slowed down, are discovered. By defining an internal state for the
excited degrees of freedom, it is shown that the drastic slowing down occurs
when the internal state is in a critical state. The relaxation dynamics brings
the internal state into the critical state, and the critical bottleneck of
relaxation is self-organized. Relevance of our result to relaxation phenomena
in condensed matters or large molecules is briefly discussed.Comment: 4pages, 5 figure
Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38
We present high contrast images of the hydrogen white dwarf G 29-38 taken in
the near infrared with the Hubble Space Telescope and the Gemini North
Telescope as part of a high contrast imaging search for substellar objects in
orbit around nearby white dwarfs.
We review the current limits on planetary companions for G29-38, the only
nearby white dwarf with an infrared excess due to a dust disk. We add our
recent observations to these limits to produce extremely tight constraints on
the types of possible companions that could be present. No objects 6
M are detected in our data at projected separations 12 AU, and no
objects 16 M are detected for separations from 3 to 12 AU, assuming
a total system age of 1 Gyr. Limits for companions at separations 3 AU come
from a combination of 2MASS photometry and previous studies of G29-38's
pulsations. Our imaging with Gemini cannot confirm a tentative claim for the
presence of a low mass brown dwarf. These observations demonstrate that a
careful combination of several techniques can probe nearby white dwarfs for
large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap
Electronic doping of graphene by deposited transition metal atoms
We perform a phenomenological analysis of the problem of the electronic
doping of a graphene sheet by deposited transition metal atoms, which aggregate
in clusters. The sample is placed in a capacitor device such that the
electronic doping of graphene can be varied by the application of a gate
voltage and such that transport measurements can be performed via the
application of a (much smaller) voltage along the graphene sample, as reported
in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows
us to explain the thermodynamic properties of the device, such as the level of
doping of graphene and the ionisation potential of the metal clusters in terms
of the chemical interaction between graphene and the clusters. We are also
able, by modelling the metallic clusters as perfect conducting spheres, to
determine the scattering potential due to these clusters on the electronic
carriers of graphene and hence the contribution of these clusters to the
resistivity of the sample. The model presented is able to explain the
measurements performed by Pi et al. on Pt-covered graphene samples at the
lowest metallic coverages measured and we also present a theoretical argument
based on the above model that explains why significant deviations from such a
theory are observed at higher levels of coverage.Comment: 16 pages, 10 figure
On the dynamics of a self-gravitating medium with random and non-random initial conditions
The dynamics of a one-dimensional self-gravitating medium, with initial
density almost uniform is studied. Numerical experiments are performed with
ordered and with Gaussian random initial conditions. The phase space portraits
are shown to be qualitatively similar to shock waves, in particular with
initial conditions of Brownian type. The PDF of the mass distribution is
investigated.Comment: Latex, figures in eps, 23 pages, 11 figures. Revised versio
The Jeans Instability in Presence of Viscous Effects
An analysis of the gravitational instability in presence of dissipative
effects is addressed. In particular, the standard Jeans Mechanism and the
generalization in treating the Universe expansion are both analyzed when bulk
viscosity affects the first-order Newtonian dynamics. As results, the
perturbation evolution is founded to be damped by dissipative processes and the
top-down mechanism of structure fragmentation is suppressed. In such a scheme,
the value of the Jeans Mass remains unchanged also in presence of viscosity.Comment: 13 pages, 2 figure
Equation of the field lines of an axisymmetric multipole with a source surface
Optical spectropolarimeters can be used to produce maps of the surface magnetic fields of stars and hence to determine how stellar magnetic fields vary with stellar mass, rotation rate, and evolutionary stage. In particular, we now can map the surface magnetic fields of forming solar-like stars, which are still contracting under gravity and are surrounded by a disk of gas and dust. Their large scale magnetic fields are almost dipolar on some stars, and there is evidence for many higher order multipole field components on other stars. The availability of new data has renewed interest in incorporating multipolar magnetic fields into models of stellar magnetospheres. I describe the basic properties of axial multipoles of arbitrary degree ℓ and derive the equation of the field lines in spherical coordinates. The spherical magnetic field components that describe the global stellar field topology are obtained analytically assuming that currents can be neglected in the region exterior to the star, and interior to some fixed spherical equipotential surface. The field components follow from the solution of Laplace’s equation for the magnetostatic potential
The Galactic IMF: origin in the combined mass distribution functions of dust grains and gas clouds
We present here a theoretical model to account for the stellar IMF as a
result of the composite behaviour of the gas and dust distribution functions.
Each of these has previously been modelled and the models tested against
observations. The model presented here implies a relation between the
characteristic size of the dust grains and the characteristic final mass of the
stars formed within the clouds containing the grains, folded with the relation
between the mass of a gas cloud and the characteristic mass of the stars formed
within it. The physical effects of dust grain size are due to equilibrium
relations between the efficiency of grains in cooling the clouds, which is a
falling function of grain size, and the efficiency of grains in catalyzing the
production of molecular hydrogen, which is a rising function of grain size. We
show that folding in the effects of grain distribution can yield a reasonable
quantitative account of the IMF, while gas cloud mass function alone cannot do
so.Comment: 8 pages, 6 figures, MNRAS accepted for publicatio
Slow relaxation to equipartition in spring-chain systems
In this study, one-dimensional systems of masses connected by springs, i.e.,
spring-chain systems, are investigated numerically. The average kinetic energy
of chain-end particles of these systems is larger than that of other particles,
which is similar to the behavior observed for systems made of masses connected
by rigid links. The energetic motion of the end particles is, however,
transient, and the system relaxes to thermal equilibrium after a while, where
the average kinetic energy of each particle is the same, that is, equipartition
of energy is achieved. This is in contrast to the case of systems made of
masses connected by rigid links, where the energetic motion of the end
particles is observed in equilibrium. The timescale of relaxation estimated by
simulation increases rapidly with increasing spring constant. The timescale is
also estimated using the Boltzmann-Jeans theory and is found to be in quite
good agreement with that obtained by the simulation
Velocity and Heat Flow in a Composite Two Fluid System
We describe the stress energy of a fluid with two unequal stresses and heat
flow in terms of two perfect fluid components. The description is in terms of
the fluid velocity overlap of the components, and makes no assumptions about
the equations of state of the perfect fluids. The description is applied to the
metrics of a conformally flat system and a black string.Comment: typos correcte
Gravito-magnetic instabilities in anisotropically expanding fluids
Gravitational instabilities in a magnetized Friedman - Robertson - Walker
(FRW) Universe, in which the magnetic field was assumed to be too weak to
destroy the isotropy of the model, are known and have been studied in the past.
Accordingly, it became evident that the external magnetic field disfavors the
perturbations' growth, suppressing the corresponding rate by an amount
proportional to its strength. However, the spatial isotropy of the FRW Universe
is not compatible with the presence of large-scale magnetic fields. Therefore,
in this article we use the general-relativistic (GR) version of the
(linearized) perturbed magnetohydrodynamic equations with and without
resistivity, to discuss a generalized Jeans criterion and the potential
formation of density condensations within a class of homogeneous and
anisotropically expanding, self-gravitating, magnetized fluids in curved
space-time. We find that, for a wide variety of anisotropic cosmological
models, gravito-magnetic instabilities can lead to sub-horizonal, magnetized
condensations. In the non-resistive case, the power spectrum of the unstable
cosmological perturbations suggests that most of the power is concentrated on
large scales (small k), very close to the horizon. On the other hand, in a
resistive medium, the critical wave-numbers so obtained, exhibit a delicate
dependence on resistivity, resulting in the reduction of the corresponding
Jeans lengths to smaller scales (well bellow the horizon) than the
non-resistive ones, while increasing the range of cosmological models which
admit such an instability.Comment: 10 pages RevTex, 4 figures, accepted for publication in the
International Journal of Modern Physics
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