1,904 research outputs found
The Solar Photospheric Nitrogen Abundance: Determination with 3D and 1D Model Atmospheres
We present a new determination of the solar nitrogen abundance making use of
3D hydrodynamical modelling of the solar photosphere, which is more physically
motivated than traditional static 1D models. We selected suitable atomic
spectral lines, relying on equivalent width measurements already existing in
the literature. For atmospheric modelling we used the co 5 bold 3D radiation
hydrodynamics code. We investigated the influence of both deviations from local
thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities
(granulation effects) on the resulting abundance. We also compared several
atlases of solar flux and centre-disc intensity presently available. As a
result of our analysis, the photospheric solar nitrogen abundance is A(N) =
7.86 +/- 0.12.Comment: 6 pages, 4 figure
Stress influence on high temperature oxide scale growth: modeling and investigation on a thermal barrier coating system.
International audienceIn thermal barrier coating (TBC) systems, an oxide layer develops at high temperature below the ceramic coating, leading at long term to the mechanical failure of the structure upon cooling. This study investigates a mechanism of stress-affected oxidation likely to induce the growth of a non-uniform oxide scale detrimental to the TBC lifetime. A continuum thermodynamics formulation is derived accounting for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. It specially includes the contributions of the large volumetric strain and the mass consumption associated with metal oxidation. A continuum mechanics/mass diffusion framework is used along with the developed formulation for the interface evolution to study the growth of an oxide layer coupled with local stress development. The implementation of the model has required the development of a specific simulation tool, based on a finite element method completed with an external routine for the phase boundary propagation. Results on an electron-beam physical vapor deposited (EB-PVD) TBC case are presented. The processes resulting in a non-uniform oxide scale growth are analyzed and the main influences are discussed
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
Recent improvements in stellar models for intermediate-mass and massive stars
are recalled, together with their expectations for the synthesis of radioactive
nuclei of lifetime Myr, in order to re-examine the origins
of now extinct radioactivities, which were alive in the solar nebula. The
Galactic inheritance broadly explains most of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly Fe require
nucleosynthesis events close to the solar formation. We outline the persisting
difficulties to account for these nuclei by Intermediate Mass Stars (2
M/M). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in Pd. This is shown for two recent
approaches to Deep Mixing. Even a late contamination by a Massive Star meets
problems. In fact, inhomogeneous addition of Supernova debris predicts
non-measured excesses on stable isotopes. Revisions invoking specific low-mass
supernovae and/or the sequential contamination of the pre-solar molecular cloud
might be affected by similar problems, although our conclusions here are
weakened by our schematic approach to the addition of SN ejecta. The limited
parameter space remaining to be explored for solving this puzzle is discussed.Comment: Accepted for publication on Ap
Modelling stress-diffusion controlled phase transformations : application to stress corrosion cracking
National audienceStress Corrosion Cracking (SCC) represents a significant cause of failure in pressurised water reactors and many efforts have been made to address this problem [1]. It involves the combined action of the environment, mechanical stresses and material properties on the damage of engineering components. Current SCC models developed to predict crack growth behaviour or SCC susceptibility criteria do not fully incorporate the complex multiphysical processes that occur during oxidation at the scale of the microstructure. The aim of the work is to formulate a multi-physics modelling framework based on continuum thermodynamics able to describe the growth of an oxide film on a polycrystalline material using the phase field method
Rebrote, rendimiento y nutrición de Leymus chinensis y Hordeum brevisubulatum en respuesta a la frecuencia e intensidad de defoliación
The effects of different defoliation intensities and frequencies were studied on regrowth and herbage mass of Leymus chinensis and Hordeum brevisubulatum in northeast China for two years. Plants were defoliated to 6, 8 or 10 cm stubble height by removing about 40% of growth down to each designated defoliation height. In the first year, L. chinensis was defoliated 22, 17 or 13 times, and in the second year was defoliated 21, 15 or 15 times to reach 6, 8 or 10 cm stubble height treatments, respectively. H. brevisubulatum was defoliated 26, 21 or 15 times in the first year, and 28, 23 or 21 times in the second year to reach the 6, 8 or 10 cm stubble, respectively. L. chinensis was more productive than H. brevisubulatum, but H. brevisubulatum showed a better forage quality than L. chinensis because H. brevisubulatum showed a higher leaf to stem ratio and crude protein concentration than L. chinensis. Both species produced the highest yield, but the lowest quality when defoliated to 10 cm stubble. There were no significant differences in water soluble carbohydrate (WSC) concentrations in below-ground culm and rhizome tissues between defoliation heights, but L. chinensis had a higher WSC concentration than H.brevisubulatum.Fil: Song, Y.. Dalian Minzu University. College of Environment and Bioresources; ChinaFil: Busso, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Yu, Y.. China National Environmental Monitoring Center; ChinaFil: Wang, P.. Northeast Normal University. School of Environment; ChinaFil: Wuyunna. Dalian Minzu University. College of Environment and Bioresources; ChinaFil: Zhou, D.. Chinese Academy of Sciences; República de Chin
Abundance gradients in the Milky Way for alpha elements, Iron peak elements, Barium, Lanthanum and Europium
We model the abundance gradients in the disk of the Milky Way for several
chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba,
La and Eu), and compare our results with the most recent and homogeneous
observational data. We adopt a chemical evolution model able to well reproduce
the main properties of the solar vicinity. We compute, for the first time, the
abundance gradients for all the above mentioned elements in the galactocentric
distance range 4 - 22 kpc. The comparison with the observed data on Cepheids in
the galactocentric distance range 5-17 kpc gives a very good agreement for many
of the studied elements. In addition, we fit very well the data for the
evolution of Lanthanum in the solar vicinity for which we present results here
for the first time. We explore, also for the first time, the behaviour of the
abundance gradients at large galactocentric distances by comparing our results
with data relative to distant open clusters and red giants and select the best
chemical evolution model model on the basis of that. We find a very good fit to
the observed abundance gradients, as traced by Cepheids, for most of the
elements, thus confirming the validity of the inside-out scenario for the
formation of the Milky Way disk as well as the adopted nucleosynthesis
prescriptions.Comment: 11 pages, 9 figures, accepted for publication in A&
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