364 research outputs found
Positive similarity solutions for a discrete velocity Boltzmann coagulation-fragmentation model
URL: http://www-spht.cea.fr/articles/T93/146International audienceWe consider the Slemrod et al coagulation-fragmentation model which is essentially the 2-dimensional Broadwell model including inelastic collisions. We construct two classes of similarity solutions (variable ), positive for : the Rankine-Hugoniot solutions and the scalar Riccati similarity solutions. Previous solutions were built up with positivity along half of the x-axis. For the two classes we determine in the parameter space, building up the solutions, domains corresponding to positive solutions
The Transparency of Solar Coronal Active Regions
Resonance scattering has often been invoked to explain the disagreement
between the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII
15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its much
higher oscillator strength, are preferentially scattered out of the line of
sight, thus reducing the observed line ratio. Recent laboratory measurements,
however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26
Angstroms, suggesting that the observed 3C/3D ratio results from blending.
Given our new understanding of the fundamental spectroscopy, we have
re-examined the original solar spectra, identifying the Fe XVI 15.21 line and
measuring its flux to account for the contribution of Fe XVI to the 15.26 flux.
Deblending brings the 3C/3D ratio into good agreement with the experimental
ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe
XVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonance
scattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstrom
images. The images must, instead, be unresolved due to the large number of
structures at this temperature. Insignificant resonance scattering implies that
future instruments with higher spatial resolution could resolve the active
region plasma into its component loop structures.Comment: accepted to Ap J Letter
Effective range function below threshold
We demonstrate that the kernel of the Lippmann-Schwinger equation, associated
with interactions consisting of a sum of the Coulomb plus a short range nuclear
potential, below threshold becomes degenerate. Taking advantage of this fact,
we present a simple method of calculating the effective range function for
negative energies. This may be useful in practice since the effective range
expansion extrapolated to threshold allows to extract low-energy scattering
parameters: the Coulomb-modified scattering length and the effective range.Comment: 14 pages, 1 figur
Extraction of scattering lengths from final-state interactions
A recently proposed method based on dispersion theory, that allows to extract
the scattering length of a hadronic two-body system from corresponding
final-state interactions, is generalized to the situation where the Coulomb
interaction is present. The steps required in a concrete practical application
are discussed in detail. In addition a thorough examination of the accuracy of
the proposed method is presented and a comparison is made with results achieved
with other methods like the Jost-function approach based on the effective-range
approximation. Deficiencies of the latter method are pointed out. The
reliability of the dispersion theory method for extracting also the effective
range is investigated.Comment: 16 pages, 6 figures, some corrections to text, to appear in Phys.
Rev.
Atomic data from the IRON Project. I. Electron-impact scattering of Fe17+ using <I>R</I>-matrix theory with intermediate coupling
We present results for electron-impact excitation of F-like Fe calculated using R-matrix theory where an intermediate-coupling frame transformation (ICFT) is used to obtain level-resolved collision strengths. Two such calculations are performed, the first expands the target using 2s2 2p5, 2s 2p6, 2s2 2p4 3l, 2s 2p5 3l, and 2p6 3l configurations while the second calculation includes the 2s2 2p4 4l, 2s 2p5 4l, and 2p6 4l configurations as well. The effect of the additional structure in the latter calculation on the n=3 resonances is explored and compared with previous calculations. We find strong resonant enhancement of the effective collision strengths to the 2s2 2p4 3s levels. A comparison with a Chandra X-ray observation of Capella shows that the n=4 R-matrix calculation leads to good agreement with observation</p
The Translation Regulatory Subunit eIF3f Controls the Kinase-Dependent mTOR Signaling Required for Muscle Differentiation and Hypertrophy in Mouse
The mTORC1 pathway is required for both the terminal muscle differentiation and
hypertrophy by controlling the mammalian translational machinery via
phosphorylation of S6K1 and 4E-BP1. mTOR and S6K1 are connected by interacting
with the eIF3 initiation complex. The regulatory subunit eIF3f plays a major
role in muscle hypertrophy and is a key target that accounts for MAFbx function
during atrophy. Here we present evidence that in MAFbx-induced atrophy the
degradation of eIF3f suppresses S6K1 activation by mTOR, whereas an eIF3f mutant
insensitive to MAFbx polyubiquitination maintained persistent phosphorylation of
S6K1 and rpS6. During terminal muscle differentiation a conserved TOS motif in
eIF3f connects mTOR/raptor complex, which phosphorylates S6K1 and regulates
downstream effectors of mTOR and Cap-dependent translation initiation. Thus
eIF3f plays a major role for proper activity of mTORC1 to regulate skeletal
muscle size
X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres
X-ray spectra from stellar coronae are reprocessed by the underlying
photosphere through scattering and photoionization events. While reprocessed
X-ray spectra reaching a distant observer are at a flux level of only a few
percent of that of the corona itself, characteristic lines formed by inner
shell photoionization of some abundant elements can be significantly stronger.
The emergent photospheric spectra are sensitive to the distance and location of
the fluorescing radiation and can provide diagnostics of coronal geometry and
abundance. Here we present Monte Carlo simulations of the photospheric Kalpha
doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source.
Fluorescent line strengths have been computed as a function of the height of
the radiation source, the temperature of the ionising X-ray spectrum, and the
viewing angle. We also illustrate how the fluorescence efficiencies scale with
the photospheric metallicity and the Fe abundance. Based on the results we make
three comments: (1) fluorescent Fe lines seen from pre-main sequence stars
mostly suggest flared disk geometries and/or super-solar disk Fe abundances;
(2) the extreme ~1400 mA line observed from a flare on V1486 Ori can be
explained entirely by X-ray fluorescence if the flare itself were partially
eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by
Swift during a large flare on II Peg is consistent with X-ray excitation and
does not require a collisional ionisation contribution. There is no convincing
evidence supporting the energetically challenging explanation of electron
impact excitation for observed stellar Fe Kalpha lines.Comment: 30 pages; accepted for publication in the Astrophysical Journa
The corona and upper transition region of epsilon Eridani
We present analyses of observations of epsilon Eridani (K2 V) made with the
Low Energy Transmission Grating Spectrometer on Chandra and the Extreme
Ultraviolet Explorer, supplemented by observations made with the Space
Telescope Imaging Spectrograph, the Far Ultraviolet Spectroscopic Explorer and
the Reflection Grating Spectrometer on XMM-Newton. The observed emission lines
are used to find relative element abundances, to place limits on the electron
densities and pressures and to determine the mean apparent emission measure
distribution. As in the previous paper by Sim & Jordan (2003a), the mean
emitting area as a function of the electron temperature is derived by
comparisons with a theoretical emission measure distribution found from energy
balance arguments. The final model has a coronal temperature of 3.4 x 10^6 K,
an electron pressure of 1.3 x 10^16 cm^-3 K at T_e = 2 x 10^5 K and an area
filling factor of 0.14 at 3.2 x 10^5 K. We discuss a number of issues
concerning the atomic data currently available. Our analyses are based mainly
on the latest version of CHIANTI (v5.2). We conclude that the Ne/O relative
abundance is 0.30, larger than that recommended from solar studies, and that
there is no convincing evidence for enhanced coronal abundances of elements
with low first ionization potentials.Comment: accepted by MNRAS; 19 pages, five figures, 10 table
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