3,950 research outputs found
Implementing TOPbase/Iron Project: Continuous Absorption from Fe II
We discuss implementation of TOPbase and Iron Project opacities for stellar
spectral codes. We use a technique employed by Peach, where a
Boltzmann-averaged cross section is calculated for selected temperatures, and
the opacity obtained from double interpolation in temperature and wavelength.
It is straightforward to include {\it all} levels for which cross sections have
been calculated. Boltzmann-averaged cross sections for Fe II show a local
maximum between 1700 and 2000[A]. We suggest this feature arises from 3d^5 4snl
to 3d^5 4pnl transitions within Fe II. IUE spectra of iron-rich CP stars show
local minima in this region. Theoretical calculations of a representative
stellar continuum demonstrate that Fe II photoionization contributes
significantly to the observed minima.Comment: 6 pages, 5 figures. See
http://www.astro.lsa.umich.edu/users/cowley/paper2r for better definition
figure
Atomic data from the Iron Project. LXIV. Radiative transition rates and collision strengths for Ca II
This work reports radiative transition rates and electron impact excitation
rate coefficients for levels of the n= 3, 4, 5, 6, 7, 8 configurations of Ca
II. The radiative data were computed using the Thomas-Fermi-Dirac central
potential method in the frozen core approximation and includes the polarization
interaction between the valence electron and the core using a model potential.
This method allows for configuration interactions (CI) and relativistic effects
in the Breit-Pauli formalism. Collision strengths in LS-coupling were
calculated in the close coupling approximation with the R-matrix method. Then,
fine structure collision strengths were obtained by means of the
intermediate-coupling frame transformation (ICFT) method which accounts for
spin-orbit coupling effects. We present extensive comparisons with the most
recent calculations and measurements for Ca II as well as a comparison between
the core polarization results and the "unpolarized" values. We find that core
polarization affects the computed lifetimes by up to 20%. Our results are in
very close agreement with recent measurements for the lifetimes of metastable
levels. The present collision strengths were integrated over a Maxwellian
distribution of electron energies and the resulting effective collision
strengths are given for a wide range of temperatures. Our effective collision
strengths for the resonance transitions are within ~11% from previous values
derived from experimental measurements, but disagree with latter computations
using the distorted wave approximation.Comment: 8 pages, 4 figures. The atomic data from this work, including energy
levels, A-values, and effective collision strengths, is available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
Time-dependent Photoionization of Gaseous Nebulae: the Pure Hydrogen Case
We study the problem of time-dependent photoionization of low density gaseous
nebulae subjected to sudden changes in the intensity of ionizing radiation. To
this end, we write a computer code that solves the full time-dependent energy
balance, ionization balance, and radiation transfer equations in a
self-consistent fashion for a simplified pure hydrogen case. It is shown that
changes in the ionizing radiation yield ionization/thermal fronts that
propagate through the cloud, but the propagation times and response times to
such fronts vary widely and non-linearly from the illuminated face of the cloud
to the ionization front (IF). Ionization/thermal fronts are often supersonic,
and in slabs initially in pressure equilibrium such fronts yield large pressure
imbalances that are likely to produce important dynamical effects in the cloud.
Further, we studied the case of periodic variations in the ionizing flux. It
is found that the physical conditions of the plasma have complex behaviors that
differ from any steady-state solutions. Moreover, even the time average
ionization and temperature is different from any steady-state case. This time
average is characterized by over-ionization and a broader IF with respect to
the steady-state solution for a mean value of the radiation flux. Around the
time average of physical conditions there is large dispersion in instantaneous
conditions, particularly across the IF, which increases with the period of
radiation flux variations. Moreover, the variations in physical conditions are
asynchronous along the slab due to the combination of non-linear propagation
times for thermal/ionization fronts and equilibration times.Comment: Accepted for publication in ApJ. 36 pages, 12 figure
On the changes in the physical properties of the ionized region around the Weigelt structures in Eta Carinae over the 5.54-yr spectroscopic cycle
We present HST/STIS observations and analysis of two prominent nebular
structures around the central source of Eta Carinae, the knots C and D. The
former is brighter than the latter for emission lines from intermediate or high
ionization potential ions. The brightness of lines from intermediate and high
ionization potential ions significantly decreases at phases around periastron.
We do not see conspicuous changes in the brightness of lines from low
ionization potential (<13.6 eV) that the total extinction towards the Weigelt
structures is that the total extinction towards the Weigelt structures is AsubV
=2/0. that the total extinction towards the Weigelt structures is AV = 2.0.
Weigelt C and D are characterized by an electron density of that the total
extinction towards the Weigelt structures is AV = 2.0. Weigelt C and D are
characterized by an electron density of 10exp6.9 cm-3 that does not
significantly change throughout the orbital cycle. The electron temperature
varies from 5500 K (around periastron) to 7200 K (around apastron). The
relative changes in the brightness of He I lines are well reproduced by the
variations in the electron temperature alone. We found that, at phases around
periastron, the electron temperature seems to be higher for Weigelt C than that
of D. The Weigelt structures are located close to the Homunculus equatorial
plane, at a distance of about 1240 AU from the central source. From the
analysis of proper motion and age, the Weigelt complex can be associated with
the equatorial structure called the Butterfly Nebula surrounding the central
binary system.Comment: 19 pages, 18 figure
[TiII] and [NiII] emission from the strontium filament of eta Carinae
We study the nature of the [TiII] and [NiII] emission from the so-called
strontium filament found in the ejecta of eta Carinae. To this purpose we
employ multilevel models of the TiII and NiII systems which are used to
investigate the physical condition of the filament and the excitation
mechanisms of the observed lines. For the TiII ion, for which no atomic data
was previously available, we carry out ab initio calculations of radiative
transition rates and electron impact excitation rate coefficients. It is found
that the observed spectrum is consistent with the lines being excited in a
mostly neutral region with an electron density of the order of cm
and a temperature around 6000 K. In analyzing three observations with different
slit orientations recorded between March~2000 and November~2001 we find line
ratios that change among various observations, in a way consistent with changes
of up to an order of magnitude in the strength of the continuum radiation
field. These changes result from different samplings of the extended filament,
due to the different slit orientations used for each observation, and yield
clues on the spatial extent and optical depth of the filament. The observed
emission indicates a large Ti/Ni abundance ratio relative to solar abundances.
It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas
fractionation processes and does not reflect the absolute Ti/Ni ratio in the
ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within
the filament and suggest that the observed gas phase overabundance of TiComment: 14 paginas, 12 figure
Superconducting niobium thin film slow-wave structures
A superconducting comb structure as a slow-wave element in a traveling-wave maser will significantly improve maser noise temperature and gain by reducing the insertion loss. The results of the insertion loss measurements of superconducting niobium slow-wave structures subjected to maser operating conditions at X-Band frequencies are presented
Atomic Processes in Planetary Nebulae and H II Regions
Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have
driven much development in atomic physics. In the last few years the
combination of a generation of powerful observatories, the development of ever
more sophisticated spectral modeling codes, and large efforts on mass
production of high quality atomic data have led to important progress in our
understanding of the atomic spectra of such astronomical objects. In this paper
I review such progress, including evaluations of atomic data by comparisons
with nebular spectra, detection of spectral lines from most iron-peak elements
and n-capture elements, observations of hyperfine emission lines and analysis
of isotopic abundances, fluorescent processes, and new techniques for
diagnosing physical conditions based on recombination spectra. The review is
directed toward atomic physicists and spectroscopists trying to establish the
current status of the atomic data and models and to know the main standing
issues.Comment: 9 pages, 1 figur
Distinguished bases of exceptional modules
Exceptional modules are tree modules. A tree module usually has many tree
bases and the corresponding coefficient quivers may look quite differently. The
aim of this note is to introduce a class of exceptional modules which have a
distinguished tree basis, we call them radiation modules (generalizing an
inductive construction considered already by Kinser). For a Dynkin quiver,
nearly all indecomposable representations turn out to be radiation modules, the
only exception is the maximal indecomposable module in case E_8. Also, the
exceptional representation of the generalized Kronecker quivers are given by
radiation modules. Consequently, with the help of Schofield induction one can
display all the exceptional modules of an arbitrary quiver in a nice way.Comment: This is a revised and slightly expanded version. Propositions 1 and 2
have been corrected, some examples have been inserte
Localization of Eosinophilic Esophagitis from H&E stained images using multispectral imaging
This study is an initial investigation on the capability of multispectral imaging to capture subtle spectral information that would enable the automatic delineation between the eosinophilic esophagitis and other eosin stained tissue components, especially the RBCs. In the method, a principal component analysis (PCA) was performed on the spectral transmittance samples of the different tissue components, excluding however the transmittance samples of the eosinophilic esophagitis. From the average spectral error configuration of the eosinophilic esophagitis transmittance samples, i.e. the difference between the actual transmittance and the estimated transmittance using m PC vectors, we indentified two spectral bands by which we can localize the eosinophils. Initial results show the possibility of automatically localizing the eosinophilic esophagitis by utilizing spectral information
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