14,172 research outputs found
Modelisation of transition and noble metal vicinal surfaces: energetics, vibrations and stability
The energetics of transition and noble metal (Rh, Pd, Cu) vicinal surfaces,
i.e., surface energy, step energy, kink energy and electronic interactions
between steps, is studied at 0K from electronic structure calculations in the
tight-binding approximation using a {\it s, p} and {\it d} valence orbital
basis set. Then, the surface phonon spectra of copper are investigated in the
harmonic approximation with the help of a semi-empirical inter-atomic
potential. This allows to derive the contribution of phonons at finite
temperatures to the step free energy and to the interactions between steps. The
last part is devoted to the stability of vicinal surfaces relative to faceting
with special attention to the domain of orientations (100)-(111).
Semi-empirical potentials are shown to be not realistic enough to give a
reliable answer to this problem. The results derived from electronic structure
calculations predict a variety of behaviors and, in particular, a possible
faceting into two other vicinal orientations. Finally, temperature effects are
discussed. Comparisons are made with other theoretical works and available
experiments
Asteroseismology of exoplanets host stars: the special case of Horologii (HD17051)
{This paper presents detailed analysis and modelisation of the star HD17051
(alias Hor), which appears as a specially interesting case among
exoplanet host stars. As most of these stars, Hor presents a
metallicity excess which has been measured by various observers who give
different results, ranging from [Fe/H] = 0.11 to 0.26, associated with
different atmospheric parameters. Meanwhile the luminosity of the star may be
determined owing to Hipparcos parallax. Although in the southern hemisphere,
this star belongs to the Hyades stream and its external parameters show that it
could even be one of the Hyades stars ejected during cluster formation. The aim
of this work was to gather and analyse our present knowledge on this star and
to prepare seismic tests for future observations with the HARPS spectrometer
(planned for November 2006).} {We have computed evolutionary tracks with
various metallicities, in the two frameworks of primordial overmetallicity and
accretion. We have concentrated on models inside the error boxes given by the
various observers in the log g - log T diagram. We then computed the
adiabatic oscillation frequencies of these models to prepare future
observations.} {The detailed analysis of Hor presented in this paper
already allowed to constrain its external parameters, mass and age. Some values
given in the literature could be rejected as inconsistent with the overall
analysis. We found that a model computed with the Hyades parameters (age,
metallicity) was clearly acceptable, but other ones were possible too. We are
confident that observations with HARPS will allow for a clear conclusion about
this star and that it will bring important new light on the physics of
exoplanet host stars.}Comment: to be published in Astronomy and Astrophysic
Infrared properties of the SDSS-maxBCG galaxy clusters
The physics of galaxy clusters has proven to be influenced by several
processes connected with their galactic component which pollutes the ICM with
metals, stars and dust. However, it is not clear whether the presence of
diffuse dust can play a role in clusters physics since a characterisation of
the IR properties of galaxy clusters is yet to be completely achieved. We focus
on the recent work of Giard et al. (2008) who performed a stacking analysis of
the IRAS data in the direction of several thousands of galaxy clusters,
providing a statistical characterisation of their IR luminosity and redshift
evolution. We model the IR properties of the galactic population of the
SDSS-maxBCG clusters (0.1<z<0.3) in order to check if it accounts for the
entire observed signal and to constrain the possible presence of other
components, like dust in the ICM. Starting from the optical properties of the
galaxy members, we estimate their emission in the 60 and 100 micron IRAS bands
making use of modeled SEDs of different spectral types (E/S0, Sa, Sb, Sc and
starburst). We also consider the evolution of the galactic
population/luminosity with redshift. Our results indicate that the galactic
emission, which is dominated by the contribution of star-forming galaxies, is
consistent with the observed signal. In fact, our model slightly overestimates
the observed fluxes, with the excess being concentrated in low-redshift
clusters (z <~ 0.17). This indicates that, if present, the IR emission from
intracluster dust must be very small. We obtain an upper limit on the
dust-to-gas mass ratio in the ICM of Z_d <~ 5 10^-5. The excess in luminosity
obtained at low redshift constitutes an indication that the cluster environment
is driving a process of star-formation quenching in its galaxy members.Comment: 12 pages, 6 figures, 2 tables. Accepted for publication in A&
Model adaptivity for finite element analysis of thin or thick plates based on equilibrated boundary stress resultants
Purpose – The purpose of this paper is to address error-controlled adaptive finite element (FE) method for thin and thick plates. A procedure is presented for determining the most suitable plate model (among available hierarchical plate models) for each particular FE of the selected mesh, that is provided as the final output of the mesh adaptivity procedure. \ud
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Design/methodology/approach – The model adaptivity procedure can be seen as an appropriate extension to model adaptivity for linear elastic plates of so-called equilibrated boundary traction approach error estimates, previously proposed for 2D/3D linear elasticity. Model error indicator is based on a posteriori element-wise computation of improved (continuous) equilibrated boundary stress resultants, and on a set of hierarchical plate models. The paper illustrates the details of proposed model adaptivity procedure for choosing between two most frequently used plate models: the one of Kirchhoff and the other of Reissner-Mindlin. The implementation details are provided for a particular case of the discrete Kirchhoff quadrilateral four-node plate FE and the corresponding Reissner-Mindlin quadrilateral with the same number of nodes. The key feature for those elements that they both provide the same quality of the discretization space (and thus the same discretization error) is the one which justifies uncoupling of the proposed model adaptivity from the mesh adaptivity. \ud
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Findings – Several numerical examples are presented in order to illustrate a very satisfying performance of the proposed methodology in guiding the final choice of the optimal model and mesh in analysis of complex plate structures. \ud
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Originality/value – The paper confirms that one can make an automatic selection of the most appropriate plate model for thin and thick plates on the basis of proposed model adaptivity procedure.\u
Evidence for speed-dependent effects in NH3 self-broadened spectra: towards a new determination of the Boltzmann constant
In this paper we present an accurate analysis of the shape of an isolated
rovibrational ammonia line from the strong nu2 band around 10 m, recorded
by laser absorption spectroscopy. Experimental spectra obtained under
controlled temperature and pressure, are confronted to various models that take
into account Dicke narrowing or speed-dependent effects. Our results show clear
evidence for speed-dependent broadening and shifting, which had never been
demonstrated so far in NH3. Accurate lineshape parameters of the nu2 saQ(6,3)
line are obtained. Our current project aiming at measuring the Boltzmann
constant, kB, by laser spectroscopy will straight away benefit from such
knowledge. We anticipate that a first optical determination of kB with a
competitive uncertainty of a few ppm is now reachable.Comment: 13 pages, 5 figures, 1 tabl
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