12,183 research outputs found
Infrared Line Emission from Planetary Nebulae. I - General Theory
General theory of infrared line emission from planetary nebul
Degenerate mixing of plasma waves on cold, magnetized single-species plasmas
In the cold-fluid dispersion relation ω = ω_p/[1+(k_⊥/k_z)^(2]1/2) for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k_⊥/k_z. As a result, for any frequency ω<ω_p, there are infinitely many degenerate waves, all having the same value of k_⊥/k_z. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr = ±(ω_p^2/ω^2-1)^(1/2). Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid
Temporal and spatial plasma wave echoes
It is shown that, if a longitudinal wave is excited in a collision-free plasma and Landau-damps away, and later a second wave is excited and also damps away, then a third wave will spontaneously appear in the plasma. This wave appears long after the first two waves have damped away at a time proportional to the interval between the first two waves, and is in that sense an echo. It is also shown that, if a wave is continuously excited at one point in a plasma and a second wave is continuously excited many Landau damping lengths from the first point, then a third wave will spontaneously appear many Landau damping lengths from the second point. Fundamentally, plasma wave echoes are possible because of the reversible nature of Landau damping. However, small-angle Coulomb collisions are very effective in destroying the echo
Einstein Radii from Binary Lensing Events
We show that the Einstein ring radius and transverse speed of a lens
projected on the source plane, and , can be
determined from the light curve of a binary-source event, followed by the
spectroscopic determination of the orbital elements of the source stars. The
determination makes use of the same principle that allows one to measure the
Einstein ring radii from finite-source effects. For the case when the orbital
period of the source stars is much longer than the Einstein time scale, , there exists a single two-fold degeneracy in determining
. However, when the degeneracy can
often be broken by making use of the binary-source system's orbital motion.
%Once , and thus are determined, one can
%distinguish self-lensing events in the Large Magellanic Cloud %from Galactic
halo events. For an identifiable 8\% of all lensing events seen toward the
Large Magellanic Cloud (LMC), one can unambiguously determine whether the
lenses are Galactic, or whether they lie in the LMC itself. The required
observations can be made after the event is over and could be carried out for
the events seen by Alcock et al.\ and Aubourg et al.. In addition, we
propose to include eclipsing binaries as sources for gravitational lensing
experiments.Comment: 18 pages, revised version, submitted to Ap
Molecular-beam epitaxy of (Zn,Mn)Se on Si(100)
We have investigated the growth by molecular-beam epitaxy of the II-VI
diluted magnetic semiconductor (Zn,Mn)Se on As-passivated Si(100) substrates.
The growth start has been optimized by using low-temperature epitaxy. Surface
properties were assessed by Nomarski and scanning electron microscopy. Optical
properties of (Zn,Mn)Se have been studied by photoluminescence and a giant
Zeeman splitting of up to 30 meV has been observed. Our observations indicate a
high crystalline quality of the epitaxial films.Comment: To be published in Applied Physics Letter
Identification of the OGLE-2003-BLG-235/MOA-2003-BLG-53 Planetary Host Star
We present the results of HST observations of the host star for the first
definitive extrasolar planet detected by microlensing. The light curve model
for this event predicts that the lens star should be separated from the source
star by ~6mas at the time of the HST images. If the lens star is a late G, K or
early M dwarf, then it will be visible in the HST images as an additional
source of light that is blended with the source image. Unless the lens and
source have exactly the same colors, its presence will also be revealed by a
systematic shift between centroids of the source plus lens in different filter
bands. The HST data indicates both of these effects: the HST source that
matches the position of the source star is 0.21 magnitudes brighter in the
ACS/HRC-F814W filter than the microlensing model predicts, and there is an
offset of ~0.7mas between the centroid of this source in the F814W and F435W
filter bands. We conclude the planetary host star has been detected in these
HST images, and this identification of the lens star enables a complete
solution of the lens system. The lens parameters are determined with a Bayesian
analysis, averaging over uncertainties in the measured parameters, interstellar
extinction, and allowing for the possibility of a binary companion to the
source star. This yields a stellar mass of M_* = 0.63(+0.07/-0.09) M_solar and
a planet mass of M_p = 2.6 (+0.8/-0.6) M_Jup at an orbital separation of 4.3
(+2.5/-0.8) AU. Thus, the lens system resembles our own Solar System, with a
planet of ~3 Jupiter-masses in a Jupiter-like orbit around a star of two-thirds
of a Solar mass. These conclusions can be tested with future HST images, which
should reveal a broadening of the blended source-plus-lens point spread
function due to the relative lens-source proper motion.Comment: 11 pages, with 3 figures. to appear in ApJ Lett (Aug 20 issue
Stationary quantum Markov process for the Wigner function
As a stochastic model for quantum mechanics we present a stationary quantum
Markov process for the time evolution of the Wigner function on a lattice phase
space Z_N x Z_N with N odd. By introducing a phase factor extension to the
phase space, each particle can be treated independently. This is an improvement
on earlier methods that require the whole distribution function to determine
the evolution of a constituent particle. The process has branching and
vanishing points, though a finite time interval can be maintained between the
branchings. The procedure to perform a simulation using the process is
presented.Comment: 12 pages, no figures; replaced with version accepted for publication
in J. Phys. A, title changed, an example adde
Complex Time Solutions with Nontrivial Topology and Multi Particle Scattering in Yang-Mills Theory
A classical solution to the Yang-Mills theory is given a new semiclassical
interpretation in terms of particle scattering. It solves the complex time
boundary value problem, which arises in the semiclassical approximation to a
multi particle transition probability in the one-instanton sector at fixed
energy. The imaginary part of the action of the solution on the complex time
contour and its topological charge obey the same relation as the self-dual
Euclidean configurations. Hence the solution is relevant for the problem of
tunneling with fermion number violation in the electroweak theory. It describes
transitions from an initial state with a smaller number of particles to a final
state with a larger number of particles. The implications of these results for
multi particle production in the electroweak theory are also discussed.Comment: 10 pgs. (LaTeX), JHU-TIPAC-93001
Extremal dynamics on complex networks: Analytic solutions
The Bak-Sneppen model displaying punctuated equilibria in biological
evolution is studied on random complex networks. By using the rate equation and
the random walk approaches, we obtain the analytic solution of the fitness
threshold to be 1/(_f+1), where _f=/ (=) in the quenched
(annealed) updating case, where is the n-th moment of the degree
distribution. Thus, the threshold is zero (finite) for the degree exponent
\gamma 3) for the quenched case in the thermodynamic limit. The
theoretical value x_c fits well to the numerical simulation data in the
annealed case only. Avalanche size, defined as the duration of successive
mutations below the threshold, exhibits a critical behavior as its distribution
follows a power law, P_a(s) ~ s^{-3/2}.Comment: 6 pages, 2 figure
Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode
We have fabricated all II-VI semiconductor resonant tunneling diodes based on
the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the
quantum well, and studied their current-voltage characteristics. When subjected
to an external magnetic field the resulting spin splitting of the levels in the
quantum well leads to a splitting of the transmission resonance into two
separate peaks. This is interpreted as evidence of tunneling transport through
spin polarized levels, and could be the first step towards a voltage controlled
spin filter.Comment: To be published in Phys. Rev. Let
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