4,557 research outputs found
Evidence for Accretion in the High-resolution X-ray Spectrum of the T Tauri Star System Hen 3-600
We present high-resolution X-ray spectra of the multiple T Tauri star system
Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph on
the Chandra X-ray Observatory. Two binary components were detected in the
zeroth-order image. Hen 3-600-A, which has a large mid-infrared excess, is a
2-3 times fainter in X-rays than Hen 3-600-B, due to a large flare on B. The
dispersed X-ray spectra of the two primary components overlap spatially;
spectral analysis was performed on the combined system. Analysis of the
individual spectra was limited to regions where the contributions of A and B
can be disentangled. This analysis results in two lines of evidence indicating
that the X-ray emission from Hen 3-600 is derived from accretion processes:
line ratios of O VII indicate that the characteristic density of its
X-ray-emitting plasma is large; a significant component of low-temperature
plasma is present and is stronger in component A. These results are consistent
with results obtained from X-ray gratings spectroscopy of more rapidly
accreting systems. All of the signatures of Hen 3-600 that are potential
diagnostics of accretion activity -- X-ray emission, UV excess, H-alpha
emission, and weak infrared excess -- suggest that its components represent a
transition phase between rapidly accreting, classical T Tauri stars and
non-accreting, weak-lined T Tauri stars.Comment: latex, 27 pages, 12 figures, 6 tables; accepted by Ap
Bivariate -distribution for transition matrix elements in Breit-Wigner to Gaussian domains of interacting particle systems
Interacting many-particle systems with a mean-field one body part plus a
chaos generating random two-body interaction having strength , exhibit
Poisson to GOE and Breit-Wigner (BW) to Gaussian transitions in level
fluctuations and strength functions with transition points marked by
and , respectively; . For these systems theory for matrix elements of one-body transition
operators is available, as valid in the Gaussian domain, with , in terms of orbitals occupation numbers, level densities and an
integral involving a bivariate Gaussian in the initial and final energies. Here
we show that, using bivariate -distribution, the theory extends below from
the Gaussian regime to the BW regime up to . This is well
tested in numerical calculations for six spinless fermions in twelve single
particle states.Comment: 7 pages, 2 figure
How to make large, void free dust clusters in dusty plasma under microgravity
Collections of micrometer sized solid particles immersed in plamsa are used
to mimic many systems from solid state and fluid physics, due to their strong
electrostatic interaction, their large inertia, and the fact that they are
large enough to be visualized with ordinary optics. On Earth, gravity restricts
the so called dusty plasma systems to thin, two-dimensional layers, unless
special experimental geometries are used, involving heated or cooled electrons,
and/or the use of dielectric materials.In micro-gravity experiments, the
formation of a dust-free void breaks the isotropy of three-dimensional dusty
plasma systems. In order to do real three-dimensional experiments, this void
has somehow to be closed. In this paper, we use a fully self-consistent fluid
model to study the closure of a void in a micro-gravity experiment, by lowering
the driving potential. The analysis goes beyond the simple description of the
virtual void, which describes the formation of a void without taking the dust
into account. We show that self-organization plays an important role in void
formation and void closure, which also allows a reversed scheme, where a
discharge is run at low driving potentials and small batches of dust are added.
No hysteresis is found this way. Finally, we compare our results to recent
experiments and find good agreement,but only when we do not take
charge-exchange collisions into account
Experimental and computational characterization of a modified GEC cell for dusty plasma experiments
A self-consistent fluid model developed for simulations of micro- gravity
dusty plasma experiments has for the first time been used to model asymmetric
dusty plasma experiments in a modified GEC reference cell with gravity. The
numerical results are directly compared with experimental data and the
experimentally determined dependence of global discharge parameters on the
applied driving potential and neutral gas pressure is found to be well matched
by the model. The local profiles important for dust particle transport are
studied and compared with experimentally determined profiles. The radial forces
in the midplane are presented for the different discharge settings. The
differences between the results obtained in the modified GEC cell and the
results first reported for the original GEC reference cell are pointed out
Finite temperature excitations of a trapped Bose gas
We present a detailed study of the temperature dependence of the condensate
and noncondensate density profiles of a Bose-condensed gas in a parabolic trap.
These quantitites are calculated self-consistently using the
Hartree-Fock-Bogoliubov equations within the Popov approximation. Below the
Bose-Einstein transition the excitation frequencies have a realtively weak
temperature dependence even though the condensate is strongly depleted. As the
condensate density goes to zero through the transition, the excitation
frequencies are strongly affected and approach the frequencies of a
noninteracting gas in the high temperature limit.Comment: 4 pages, Latex, 4 postscript figures. Submitted to Physical Review
Letter
A Geophysical Atlas for Interpretation of Satellite-derived Data
A compilation of maps of global geophysical and geological data plotted on a common scale and projection is presented. The maps include satellite gravity, magnetic, seismic, volcanic, tectonic activity, and mantle velocity anomaly data. The Bibliographic references for all maps are included
Kinetic Theory of Collective Excitations and Damping in Bose-Einstein Condensed Gases
We calculate the frequencies and damping rates of the low-lying collective
modes of a Bose-Einstein condensed gas at nonzero temperature. We use a complex
nonlinear Schr\"odinger equation to determine the dynamics of the condensate
atoms, and couple it to a Boltzmann equation for the noncondensate atoms. In
this manner we take into account both collisions between
noncondensate-noncondensate and condensate-noncondensate atoms. We solve the
linear response of these equations, using a time-dependent gaussian trial
function for the condensate wave function and a truncated power expansion for
the deviation function of the thermal cloud. As a result, our calculation turns
out to be characterized by two dimensionless parameters proportional to the
noncondensate-noncondensate and condensate-noncondensate mean collision times.
We find in general quite good agreement with experiment, both for the
frequencies and damping of the collective modes.Comment: 10 pages, 8 figure
A Gapless Theory of Bose-Einstein Condensation in Dilute Gases at Finite Temperature
In this paper we develop a gapless theory of BEC which can be applied to both
trapped and homogeneous gases at zero and finite temperature. The many-body
Hamiltonian for the system is written in a form which is approximately
quadratic with higher order cubic and quartic terms. The quadratic part is
diagonalized exactly by transforming to a quasiparticle basis, while the
non-quadratic terms are dealt with using first and second order perturbation
theory. The conventional treatment of these terms, based on factorization
approximations, is shown to be inconsistent.
Infra-red divergences can appear in individual terms of the perturbation
expansion, but we show analytically that the total contribution beyond
quadratic order is finite. The resulting excitation spectrum is gapless and the
energy shifts are small for a dilute gas away from the critical region,
justifying the use of perturbation theory. Ultra-violet divergences can appear
if a contact potential is used to describe particle interactions. We show that
the use of this potential as an approximation to the two-body T-matrix leads
naturally to a high-energy renormalization.
The theory developed in this paper is therefore well-defined at both low and
high energy and provides a systematic description of Bose-Einstein condensation
in dilute gases. It can therefore be used to calculate the energies and decay
rates of the excitations of the system at temperatures approaching the phase
transition.Comment: 39 pages of Revtex. 1 figur
Vortex stabilization in Bose-Einstein condensate of alkali atom gas
A quantized vortex in the Bose-Einstein condensation (BEC), which is known to
be unstable intrinsically, is demonstrated theoretically to be stabilized by
the finite temperature effect. The mean-field calculation of Popov
approximation within the Bogoliubov theory is employed, giving rise to a
self-consistent solution for BEC confined by a harmonic potential. Physical
origin of this vortex stabilization is investigated. An equivalent effect is
also proved to be induced by an additional pinning potential at the vortex
center produced by a focused laser beam even at the lowest temperature. The
self-consistent solutions give detailed properties of a stable vortex, such as
the spatial profiles of the condensate and non-condensate, the particle current
density around the core, the whole excitation spectrum, and their temperature
dependences.Comment: 11 pages, 17 eps figure
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