1,416 research outputs found
Roundabout relaxation: collective excitation requires a detour to equilibrium
Relaxation to equilibrium after strong and collective excitation is studied,
by using a Hamiltonian dynamical system of one dimensional XY model. After an
excitation of a domain of elements, the excitation is concentrated to fewer
elements, which are made farther away from equilibrium, and the excitation
intensity increases logarithmically with . Equilibrium is reached only after
taking this ``roundabout'' route, with the time for relaxation diverging
asymptotically as with .Comment: 4 pages, 5 figure
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
Self-consistent models of cuspy triaxial galaxies with dark matter haloes
We have constructed realistic, self-consistent models of triaxial elliptical
galaxies embedded in triaxial dark matter haloes. We examined three different
models for the shape of the dark matter halo: (i) the same axis ratios as the
luminous matter (0.7:0.86:1); (ii) a more prolate shape (0.5:0.66:1); (iii) a
more oblate shape (0.7:0.93:1). The models were obtained by means of the
standard orbital superposition technique introduced by Schwarzschild.
Self-consistent solutions were found in each of the three cases. Chaotic orbits
were found to be important in all of the models,and their presence was shown to
imply a possible slow evolution of the shapes of the haloes. Our results
demonstrate for the first time that triaxial dark matter haloes can co-exist
with triaxial galaxies.Comment: Latex paper based on the AASTEX format, 20 pages, 11 figures, 2
tables. Paper submitted to Ap
Direct Dark Matter Detection with Velocity Distribution in the Eddington approach
Exotic dark matter together with the vacuum energy (associated with the
cosmological constant) seem to dominate the Universe. Thus its direct detection
is central to particle physics and cosmology. Supersymmetry provides a natural
dark matter candidate, the lightest supersymmetric particle (LSP). One
essential ingredient in obtaining the direct detection rates is the density and
the velocity distribution of the LSP in our vicinity. In the present paper we
study simultaneously density profiles and velocity distributions in the context
of the Eddington approach. In such an approach, unlike the commonly assumed
Maxwell-Boltzmann (M-B) distribution, the upper bound of the velocity arises
naturally from the potential.Comment: 21 LaTex pages, 27 figure
A Near-Infrared Spectroscopic Study of the Accreting Magnetic White Dwarf SDSS J121209.31+013627.7 and its Substellar Companion
The nature of the excess near-infrared emission associated with the magnetic
white dwarf commonly known as SDSS 1212 is investigated primarily through
spectroscopy, and also via photometry. The inferred low mass secondary in this
system has been previously detected by the emission and variation of H,
and the m spectral data presented here are consistent with the
presence of a late L or early T dwarf. The excess flux seen beyond 1.5 m
in the phase-averaged spectrum is adequately modeled with an L8 dwarf
substellar companion and cyclotron emission in a 7 MG magnetic field. This
interesting system manifests several observational properties typical of
polars, and is most likely an old interacting binary with a magnetic white
dwarf and a substellar donor in an extended low state.Comment: 28 pages, 5 figures, Accepted to Ap
On the Gravitational Collapse of a Gas Cloud in Presence of Bulk Viscosity
We analyze the effects induced by the bulk viscosity on the dynamics
associated to the extreme gravitational collapse. Aim of the work is to
investigate whether the presence of viscous corrections to the evolution of a
collapsing gas cloud influence the fragmentation process. To this end we study
the dynamics of a uniform and spherically symmetric cloud with corrections due
to the negative pressure contribution associated to the bulk viscosity
phenomenology. Within the framework of a Newtonian approach (whose range of
validity is outlined), we extend to the viscous case either the Lagrangian,
either the Eulerian motion of the system and we treat the asymptotic evolution
in correspondence to a viscosity coefficient of the form ( being the cloud density and ). We show how,
in the adiabatic-like behavior of the gas (i.e. when the politropic index takes
values ), density contrasts acquire, asymptotically, a
vanishing behavior which prevents the formation of sub-structures. We can
conclude that in the adiabatic-like collapse the top down mechanism of
structures formation is suppressed as soon as enough strong viscous effects are
taken into account. Such a feature is not present in the isothermal-like (i.e.
) collapse because the sub-structures formation is yet present
and outlines the same behavior as in the non-viscous case. We emphasize that in
the adiabatic-like collapse the bulk viscosity is also responsible for the
appearance of a threshold scale beyond which perturbations begin to increase.Comment: 13 pages, no figur
The evolution and energetics of large amplitude nonlinear internal waves on the Portuguese shelf
Intensive in-situ observations of nonlinear internal waves on the Portuguese shelf were made in August 1994, including measurements of the same wavepacket at three locations as it propagated on-shelf. The waves were characterized by sudden isotherm depressions of up to 45 m lasting 10-35 minutes, accompanied by current surges of up to 0.45 m s-1 and shears of up to 0.7 m s-1 over 60 m. The waves propagated away from the shelf break with an estimated phase speed of 0.57 m s-1. The amplitude of the waves was comparable to the theoretical maximum. The kinetic and potential energies of the waves were calculated directly from the observed current and density structures. Individual waves were associated with a total energy of up to approximately 3.0 M J m-1 per unit crest length. The depth integrated on-shelf internal wave energy flux approached 2000 W m-1 per unit crest length near the shelf break and decreased on-shore at a depth integrated rate of approximately 7.7 × 10-2 W m-2. The internal waves provided an important source of vertical mixing where they occurred on the Portuguese shelf throughout most of August 1994, represented by a sustained vertical eddy diffusivity profile which peaked at Kz ≈ 2.2 × 10-3 m2 s-1 at 30 m depth. Internal mixing would have to be parameterized by such a Kz profile in any model of the Portuguese shelf in which the stratification were to be accurately represented
Implicit Solutions of PDE's
Further investigations of implicit solutions to non-linear partial
differential equations are pursued. Of particular interest are the equations
which are Lorentz invariant. The question of which differential equations of
second order for a single unknown are solved by the imposition of an
inhomogeneous quadratic relationship among the independent variables, whose
coefficients are functions of is discussed, and it is shown that if the
discriminant of the quadratic vanishes, then an implicit solution of the
so-called Universal Field Equation is obtained. The relation to the general
solution is discussed.Comment: 11 pages LaTeX2
Unstable Disk Galaxies. I. Modal Properties
I utilize the Petrov-Galerkin formulation and develop a new method for
solving the unsteady collisionless Boltzmann equation in both the linear and
nonlinear regimes. In the first order approximation, the method reduces to a
linear eigenvalue problem which is solved using standard numerical methods. I
apply the method to the dynamics of a model stellar disk which is embedded in
the field of a soft-centered logarithmic potential. The outcome is the full
spectrum of eigenfrequencies and their conjugate normal modes for prescribed
azimuthal wavenumbers. The results show that the fundamental bar mode is
isolated in the frequency space while spiral modes belong to discrete families
that bifurcate from the continuous family of van Kampen modes. The population
of spiral modes in the bifurcating family increases by cooling the disk and
declines by increasing the fraction of dark to luminous matter. It is shown
that the variety of unstable modes is controlled by the shape of the dark
matter density profile.Comment: Accepted for publication in The Astrophysical Journa
- …