73 research outputs found
Exploring the Energy Landscape of the Charge Transport Levels in Organic Semiconductors at the Molecular Scale
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Biochemical Diagnosis of a Fatal Case of GĂŒntherâs Disease in a Newborn with Hydrops Foetalis
Peer Reviewe
Electron Impact Excitation Cross Sections for Hydrogen-Like Ions
We present cross sections for electron-impact-induced transitions n --> n' in
hydrogen-like ions C 5+, Ne 9+, Al 12+, and Ar 17+. The cross sections are
computed by Coulomb-Born with exchange and normalization (CBE) method for all
transitions with n < n' < 7 and by convergent close-coupling (CCC) method for
transitions with n 2s and 1s
--> 2p are presented as well. The CCC and CBE cross sections agree to better
than 10% with each other and with earlier close-coupling results (available for
transition 1 --> 2 only). Analytical expression for n --> n' cross sections and
semiempirical formulae are discussed.Comment: RevTeX, 5 pages, 13 PostScript figures, submitted to Phys. Rev.
On inelastic hydrogen atom collisions in stellar atmospheres
The influence of inelastic hydrogen atom collisions on non-LTE spectral line
formation has been, and remains to be, a significant source of uncertainty for
stellar abundance analyses, due to the difficulty in obtaining accurate data
for low-energy atomic collisions either experimentally or theoretically. For
lack of a better alternative, the classical "Drawin formula" is often used.
Over recent decades, our understanding of these collisions has improved
markedly, predominantly through a number of detailed quantum mechanical
calculations. In this paper, the Drawin formula is compared with the quantum
mechanical calculations both in terms of the underlying physics and the
resulting rate coefficients. It is shown that the Drawin formula does not
contain the essential physics behind direct excitation by H atom collisions,
the important physical mechanism being quantum mechanical in character.
Quantitatively, the Drawin formula compares poorly with the results of the
available quantum mechanical calculations, usually significantly overestimating
the collision rates by amounts that vary markedly between transitions.Comment: 9 pages, 6 figures, accepted for A&
Non-LTE Model Atom Construction
Model atoms are an integral part in the solution of non-LTE problems. They
comprise the atomic input data that are used to specify the statistical
equilibrium equations and the opacities and emissivities of radiative transfer.
A realistic implementation of the structure and the processes governing the
quantum-mechanical system of an atom is decisive for the successful modelling
of observed spectra. We provide guidelines and suggestions for the construction
of robust and comprehensive model atoms as required in non-LTE line-formation
computations for stellar atmospheres. Emphasis is given on the use of standard
stars for testing model atoms under a wide range of plasma conditions.Comment: 19 pages, 8 figure
Dynamics and Radiation of Young Type-Ia Supernova Remnants: Important Physical Processes
We examine and analyze the physical processes that should be taken into
account when modeling young type-Ia SNRs, with ages of several hundred years.
It is shown, that energy losses in the metal-rich ejecta can be essential for
remnants already at this stage of evolution. The influence of electron thermal
conduction and the rate of the energy exchange between electrons and ions on
the temperature distribution and the X-radiation from such remnants is studied.
The data for Tycho SNR from the XMM-Newton X-ray telescope have been employed
for the comparison of calculations with observations.Comment: 19 pages, 8 figure
Carbon abundances of early B-type stars in the solar vicinity. Non-LTE line-formation for C II/III/IV and self-consistent atmospheric parameters
Precise determinations of the chemical composition in early B-type stars
consitute fundamental observational constraints on stellar and galactochemical
evolution. Carbon is one of the most abundant metals in the Universe but
analyses in early-type stars show inconclusive results, like large
discrepancies between analyses of different lines in C II, a failure to
establish the C II/III ionization balance and the derivation of systematically
lower abundances than from other objects. We present a comprehensive and robust
C II/III/IV model for non-LTE line-formation calculations based on carefully
selected atomic data. The model is calibrated with high-S/N spectra of six
apparently slow-rotating early B-type dwarfs and giants, which cover a wide
parameter range and are randomly distributed in the solar neighbourhood. A
self-consistent quantitative spectrum analysis is performed using an extensive
iteration scheme to determine stellar atmospheric parameters and to select the
appropriate atomic data used for the derivation of chemical abundances. We
establish the carbon ionization balance for all sample stars based on a unique
set of input atomic data, achieving consistency for all modelled lines. Highly
accurate atmospheric parameters and a homogeneous carbon abundance with reduced
systematic errors are derived. This results in a present-day stellar carbon
abundance in the solar neighbourhood, which is in good agreement with recent
determinations of the solar value and with the gas-phase abundance of the Orion
H II region. The homogeneous present-day carbon abundance also conforms with
predictions of chemical-evolution models for the Galaxy. The present approach
allows us to constrain the effects of systematic errors on fundamental
parameters and abundances. (abridged)Comment: 25 pages, 22 figures. Accepted for publication in A&
Safety of artemether-lumefantrine in pregnant women with malaria: results of a prospective cohort study in Zambia
Contributions à la prévention des malformations congénitales
Doctorat en sciences médicalesinfo:eu-repo/semantics/nonPublishe
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