27,591 research outputs found
Revisiting He-like X-ray Emission Line Plasma Diagnostics
A complete model of helium-like line and continuum emission has been
incorporated into the plasma simulation code Cloudy. All elements between He
and Zn are treated, any number of levels can be considered, and radiative and
collisional processes are included. This includes photoionization from all
levels, line transfer including continuum pumping and destruction by background
opacities, scattering, and collisional processes. The model is calculated
self-consistently along with the ionization and thermal structure of the
surrounding nebula. The result is a complete line and continuum spectrum of the
plasma. Here we focus on the ions of the He I sequence and reconsider the
standard helium-like X-ray diagnostics. We first consider semi-analytical
predictions and compare these with previous work in the low-density,
optically-thin limit. We then perform numerical calculations of helium-like
X-ray emission (such as is observed in some regions of Seyferts) and predict
line ratios as a function of ionizing flux, hydrogen density, and column
density. In particular, we demonstrate that, in photoionized plasmas, the
-ratio, a density indicator in a collisional plasma, depends on the
ionization fraction and is strongly affected by optical depth for large column
densities. We also introduce the notion that the -ratio is a measure of the
incident continuum at UV wavelengths. The -ratio, which is
temperature-sensitive in a collisional plasma, is also discussed, and shown to
be strongly affected by continuum pumping and optical depth as well. These
distinguish a photoionized plasma from the more commonly studied collisional
case.Comment: 28 pages, 7 figures, accepted to Ap
Design considerations for a LORAN-C timing receiver in a hostile signal to noise environment
The environment in which a LORAN-C Timing Receiver may function effectively depends to a large extent on the techniques utilized to insure that interfering signals within the pass band of the unit are neutralized. The baseline performance manually operated timing receivers is discussed and the basic design considerations and necessary parameters for an automatic unit utilizing today's technology are established. Actual performance data is presented comparing the results obtained from a present generation timing receiver against a new generation microprocessor controlled automatic acquisition receiver. The achievements possible in a wide range of signal to noise situations are demonstrated
Improved He I Emissivities in the Case B Approximation
We update our prior work on the case B collisional-recombination spectrum of
He I to incorporate \textit{ab initio} photoionisation cross-sections. This
large set of accurate, self-consistent cross-sections represents a significant
improvement in He I emissivity calculations because it largely obviates the
piecemeal nature that has marked all modern works. A second, more recent set of
\textit{ab initio} cross-sections is also available, but we show that those are
less consistent with bound-bound transition probabilities than our adopted set.
We compare our new effective recombination coefficients with our prior work and
our new emissivities with those by other researchers, and we conclude with
brief remarks on the effects of the present work on the He I error budget. Our
calculations cover temperatures K and densities cm. Full results are available online.Comment: Accepted to MNRAS Letters; 4 pages, 4 figures, 2 tables, 1
supplemental fil
Dynamics and Manipulation of Matter-Wave Solitons in Optical Superlattices
We analyze the existence and stability of bright, dark, and gap matter-wave
solitons in optical superlattices. Then, using these properties, we show that
(time-dependent) ``dynamical superlattices'' can be used to controllably place,
guide, and manipulate these solitons. In particular, we use numerical
experiments to displace solitons by turning on a secondary lattice structure,
transfer solitons from one location to another by shifting one superlattice
substructure relative to the other, and implement solitonic ``path-following'',
in which a matter wave follows the time-dependent lattice substructure into
oscillatory motion.Comment: 6 pages, revtex, 6 figures, to appear in Physics Letters A; minor
modifications from last versio
Accurate determination of the free-free Gaunt factor. II - relativistic Gaunt factors
When modelling an ionised plasma, all spectral synthesis codes need the
thermally averaged free-free Gaunt factor defined over a very wide range of
parameter space in order to produce an accurate prediction for the spectrum.
Until now no data set exists that would meet these needs completely. We have
therefore produced a table of relativistic Gaunt factors over a much wider
range of parameter space than has ever been produced before. We present tables
of the thermally averaged Gaunt factor covering the range log10(gamma^2) = -6
to 10 and log10(u) = -16 to 13 for all atomic numbers Z = 1 through 36. The
data were calculated using the relativistic Bethe-Heitler-Elwert (BHE)
approximation and were subsequently merged with accurate non-relativistic
results in those parts of the parameter space where the BHE approximation is
not valid. These data will be incorporated in the next major release of the
spectral synthesis code Cloudy. We also produced tables of the frequency
integrated Gaunt factor covering the parameter space log10(gamma^2) = -6 to 10
for all values of Z between 1 and 36. All the data presented in this paper are
available online.Comment: 8 pages, 8 figures, 2 table
Concept design and alternate arrangements of orbiter mid-deck habitability features
The evaluations and recommendations for habitability features in the space shuttle orbiter mid-deck are summarized. The orbiter mission plans, the mid-deck dimensions and baseline arrangements along with crew compliments and typical activities were defined. Female and male anthropometric data based on zero-g operations were also defined. Evaluations of baseline and alternate feasible concepts provided several recommendations which are discussed
Radiative cooling in collisionally and photo ionized plasmas
We discuss recent improvements in the calculation of the radiative cooling in
both collisionally and photo ionized plasmas. We are extending the spectral
simulation code Cloudy so that as much as possible of the underlying atomic
data is taken from external databases, some created by others, some developed
by the Cloudy team. This paper focuses on recent changes in the treatment of
many stages of ionization of iron, and discusses its extensions to other
elements. The H-like and He-like ions are treated in the iso-electronic
approach described previously. Fe II is a special case treated with a large
model atom. Here we focus on Fe III through Fe XXIV, ions which are important
contributors to the radiative cooling of hot, 1e5 to 1e7 K, plasmas and for
X-ray spectroscopy. We use the Chianti atomic database to greatly expand the
number of transitions in the cooling function. Chianti only includes lines that
have atomic data computed by sophisticated methods. This limits the line list
to lower excitation, longer wavelength, transitions. We had previously included
lines from the Opacity Project database, which tends to include higher energy,
shorter wavelength, transitions. These were combined with various forms of the
g-bar approximation, a highly approximate method of estimating collision rates.
For several iron ions the two databases are almost entirely complementary. We
adopt a hybrid approach in which we use Chianti where possible, supplemented by
lines from the Opacity Project for shorter wavelength transitions. The total
cooling including the lightest thirty elements differs significantly from some
previous calculations
A Theory of Errors in Quantum Measurement
It is common to model random errors in a classical measurement by the normal
(Gaussian) distribution, because of the central limit theorem. In the quantum
theory, the analogous hypothesis is that the matrix elements of the error in an
observable are distributed normally. We obtain the probability distribution
this implies for the outcome of a measurement, exactly for the case of 2x2
matrices and in the steepest descent approximation in general. Due to the
phenomenon of `level repulsion', the probability distributions obtained are
quite different from the Gaussian.Comment: Based on talk at "Spacetime and Fundamental Interactions: Quantum
Aspects" A conference to honor A. P. Balachandran's 65th Birthda
Analytical study of a free-wing/free-trimmer concept
The free-wing/free-trimmer is a NASA-Conceived extension of the free-wing concept intended to permit the use of high-lift flaps. Wing pitching moments are balanced by a smaller, external surface attached by a boom or equivalent structure. The external trimmer is, itself, a miniature free wing, and pitch control of the wing-trimmer assembly is effected through a trailing-edge control tab on the trimmer surface. The longitudinal behavior of representative small free-wing/free-trimmer aircraft was analyzed. Aft-mounted trimmer surfaces are found to be superior to forward trimmers, although the permissible trimmer moment arm is limited, in both cases, by adverse dynamic effects. Aft-trimmer configurations provide excellent gust alleviation and meet fundamental stick-fixed stability criteria while exceeding the lift capabilities of pure free-wing configurations
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