92 research outputs found
Biochemical Diagnosis of a Fatal Case of Günther’s 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.
Electron impact excitation cross sections for allowed transitions in atoms
We present a semiempirical Gaunt factor for widely used Van Regemorter
formula [Astrophys. J. 136, 906 (1962)] for the case of allowed transitions in
atoms with the LS coupling scheme. Cross sections calculated using this Gaunt
factor agree with measured cross sections to within the experimental error.Comment: RevTeX, 3 pages, 10 PS figures, 2 PS tables, submitted to Phys. Rev.
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
Formation of Zr I and II lines under non-LTE conditions of stellar atmospheres
The non-local thermodynaic equilibrium (non-LTE) line formation for the two
ions of zirconium is considered through a range of spectral types when the Zr
abundance varies from the solar value down to [Zr/H] = -3. The model atom was
built using 148 energy levels of Zr I, 772 levels of Zr II, and the ground
state of Zr III. It was shown that the main non-LTE mechnism for the minority
species Zr I is ultraviolet overionization. Non-LTE leads to systematically
depleted total absorption in the Zr I lines and positive abundance corrections,
reaching to 0.33 dex for the solar metallicity models. The excited levels of Zr
II are overpopulated relative to their thermodynamic equilibrium populations in
the line formation layers due to radiative pumping from the low-excitation
levels. As a result, the line source function exceeds the Planck function
leading to weakening the Zr II lines and positive non-LTE abundance
corrections. Such corrections grow towards lower metallicity and lower surface
gravity and reach to 0.34 dex for Teff = 5500 K, log g = 2.0, [M/H] = -2. As a
test and first application of the Zr I-Zr II model atom, Zr abundance was
determined for the Sun on the basis of 1D LTE model atmosphere. Lines of Zr I
and Zr II give consistent within the error bars non-LTE abundances, while the
difference in LTE abundances amounts to 0.28 dex. The solar abundance of
zirconium obtained with the MAFAGS solar model atmosphere is log eps(Zr) =
2.63+-0.07.Comment: published in Astron. Letters, 36, 664 (2010); Erratum was submitte
Spectra of supernovae in the nebular phase
When supernovae enter the nebular phase after a few months, they reveal
spectral fingerprints of their deep interiors, glowing by radioactivity
produced in the explosion. We are given a unique opportunity to see what an
exploded star looks like inside. The line profiles and luminosities encode
information about physical conditions, explosive and hydrostatic
nucleosynthesis, and ejecta morphology, which link to the progenitor properties
and the explosion mechanism. Here, the fundamental properties of spectral
formation of supernovae in the nebular phase are reviewed. The formalism
between ejecta morphology and line profile shapes is derived, including effects
of scattering and absorption. Line luminosity expressions are derived in
various physical limits, with examples of applications from the literature. The
physical processes at work in the supernova ejecta, including gamma-ray
deposition, non-thermal electron degradation, ionization and excitation, and
radiative transfer are described and linked to the computation and application
of advanced spectral models. Some of the results derived so far from
nebular-phase supernova analysis are discussed.Comment: Book chapter for 'Handbook of Supernovae,' edited by Alsabti and
Murdin, Springer. 51 pages, 14 figure
Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres
We present the implementation of a radiative transfer solver with coherent
scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD)
simulations of stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and improved
resolution of hydrodynamical structures. We apply the code to simulate the
surface granulation in a solar-type star, ignoring magnetic fields, and
investigate the importance of coherent scattering for the atmospheric
structure. A scattering term is added to the radiative transfer equation,
requiring an iterative computation of the radiation field. We use a
short-characteristics-based Gauss-Seidel acceleration scheme to compute
radiative flux divergences for the energy equation. The effects of coherent
scattering are tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star: without
scattering, with continuum scattering only, and with both continuum and line
scattering. We show that continuum scattering does not have a significant
impact on the photospheric temperature structure for a star like the Sun.
Including scattering in line-blanketing, however, leads to a decrease of
temperatures by about 350\,K below log tau < -4. The effect is opposite to that
of 1D hydrostatic models in radiative equilibrium, where scattering reduces the
cooling effect of strong LTE lines in the higher layers of the photosphere.
Coherent line scattering also changes the temperature distribution in the high
atmosphere, where we observe stronger fluctuations compared to a treatment of
lines as true absorbers.Comment: A&A, in pres
The GALAH Survey : Non-LTE departure coefficients for large spectroscopic surveys
19 pages, 25 figures, 2 tables, arXiv abstract abridged; accepted for publication in A&AMassive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 1D MARCS model atmospheres that spans , , and . We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of stars from GALAH DR3. We found that relaxing LTE can change the abundances by between and for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the versus plane by up to , and it can remove spurious differences between the dwarfs and giants by up to . The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy.Peer reviewe
Light absorption by a collisional system
Perturbation theory is applied to the interaction between a binary collisional system and a weak radiation field to obtain the rate of induced absorption or the line profile of an optically allowed transition at low density. The collisional system wave functions are not affected by the radiation field and appropriate different expansions of these functions must be used for the two electron-atom or atom-atom systems under study. Unified expressions of the profile are given which converges to the impact limit at small detunings. The semi-classical approach of the collisional problem is given to underline the correspondence with the usual autocorrelation formalism. Both approaches break down for large detunings where a quantum collisional theory must be used.La théorie des perturbations appliquée à l'étude de l'interaction entre un champ faible et un système collisionnel binaire permet d'obtenir l'expression du taux d'absorption induit par collisions ou le profil spectral d'une transition optiquement permise à faible densité. Les fonctions d'onde du système collisionnel ne sont pas affectées par le champ radiatif et différents développements de ces fonctions sont utilisés suivant la nature du système: atome-électron ou atome-atome. Des expressions unifiées du profil sont données qui convergent vers la limite « impact » pour les petits désaccords Δω. L'approche semi-classique du problème collisionnel est donnée pour établir la correspondance avec le formalisme de la fonction d'autocorrélation. Aux grands désaccords ces deux approches sont en défaut et il est nécessaire d'utiliser un modèle collisionnel quantique
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