393 research outputs found
Calibrations of alpha Cen A & B
Detailed evolutionary models of the visual binary alpha Centauri, including
pre main-sequence evolution, have been performed using the masses recently
determined by Pourbaix et al. (1999). Models have been constructed using the
CEFF equation of state, OPAL opacities, NACRE thermonuclear reaction rates and
microscopic diffusion. A chi^2-minimization is performed to derive the most
reliable set of modeling parameters wp={t_alpha Cen, Yi, [Fe/H]i, alpha A,
alpha B}, where t alpha Cen is the age of the system, Yi the initial helium
content, [Fe/H]i the initial metallicity and, alpha A and alpha B the
convection parameters of the two components. Using the basic Bohm-Vitense
(1958) mixing-length theory of convection, we derive wp BV={2710 Myr,
0.284,0.257, 1.53, 1.57}. We obtain a noticeably smaller age than estimated
previously, in agreement with Pourbaix et al. (1999), mainly because of the
larger masses. If convective core overshoot is considered we get wp ov={3530
Myr, 0.279,0.264,1.64,1.66}. The use of Canuto & Mazitelli (1991, 1992)
convection theory leads to the set wp CM={4086 Myr, 0.271, 0.264, 0.964,
0.986}. Using the observational constraints adopted by Guenther & Demarque
(2000), and the basic mixing-length theory, we obtain wp GD={5640 Myr, 0.300,
0.296, 1.86, 1.97} and surface lithium depletions close to their observed
values.
A seismological analysis of our calibrated models has been performed. The
determination of large and small spacings between the frequencies of acoustic
oscillations from seismic observations would help to discriminate between the
models of alpha Cen computed with different masses and to confirm or rules out
the new determination of masses.Comment: accepted for publication by A&
Asteroseismology and calibration of alpha Cen binary system
Using the oscillation frequencies of alpha Cen A recently discovered by
Bouchy & Carrier, the available astrometric, photometric and spectroscopic
data, we tried to improve the calibration of the visual binary system alpha
Cen. With the revisited masses of Pourbaix et al. (2002) we do not succeed to
obtain a solution satisfying all the seismic observational constraints.
Relaxing the constraints on the masses, we have found an age t_alpha
Cen=4850+-500 Myr, an initial helium mass fraction Y_i = 0.300+-0.008, and an
initial metallicity (Z/X)_i=0.0459+-0.0019, with M_A=1.100+-0.006M_o and
M_B=0.907+-0.006M_o for alpha Cen A&B.Comment: accepted for publication as a letter in A&
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
VINCI / VLTI observations of Main Sequence stars
Main Sequence (MS) stars are by far the most numerous class in the Universe.
They are often somewhat neglected as they are relatively quiet objects (but
exceptions exist), though they bear testimony of the past and future of our
Sun. An important characteristic of the MS stars, particularly the solar-type
ones, is that they host the large majority of the known extrasolar planets.
Moreover, at the bottom of the MS, the red M dwarfs pave the way to
understanding the physics of brown dwarfs and giant planets. We have measured
very precise angular diameters from recent VINCI/VLTI interferometric
observations of a number of MS stars in the K band, with spectral types between
A1V and M5.5V. They already cover a wide range of effective temperatures and
radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as
well as the asteroseismic information available for some of these stars, the
angular diameters put strong constraints on the detailed models of these stars,
and therefore on the physical processes at play.Comment: 5 pages, 3 figures. To appear in the Proceedings of IAU Symposium
219, "Stars as Suns", Editors A. Benz & A. Dupree, Astronomical Society of
the Pacifi
Congenital tibial deficiencies: Treatment using the Ilizarov's external fixator
SummaryIntroductionCongenital longitudinal deficiency of the tibia is a rare and often syndromic anomaly. Amputation is usually the preferred treatment option in complete absence of the tibia; however, a conservative management might be implemented in partial forms or in case of amputation refusal. Our experience with the Ilizarov fixator, convinced us this device was the best suited for progressive correction of lower limbs length discrepancies and articular or bone angular limb deformities (ALD). The aim of this study is to highlight the interest of the Ilizarov fixator in the multistage conservative treatment of congenital tibial deficiencies.Material and methodsA retrospective study was conducted in nine patients suffering from Type I or II congenital tibial deficiencies (Jones) and sequentially managed using the Ilizarov technique. The functional outcome after treatment completion was then clinically assessed.ResultsThe different stages of correction were recorded for each individual patient. Patients were assessed at a mean follow-up of 18,3 years (4â32 years). The mean maximum knee flexion was 35° (0°â90°) in type I deficiencies and 118° (90°â140°) in type II deficiencies. One patient underwent amputation and a bilateral knee arthrodesis was performed in another case.DiscussionFew series in the literature report a comparable length of follow-up period in the conservative management of severe congenital tibial deficiencies. In our study, the Ilizarov fixator provided satisfactory progressive corrections of severe congenital tibial deficiencies.Level of EvidenceLevel IV therapeutic retrospective study
SynthÚse et caractérisation du quaternaire Cu2ZnSnS4 (CZTS) par voie sol-gel associé au spin coating
Les couches minces de Cu2ZnSnS4 (CZTS) ont Ă©tĂ© prĂ©parĂ©es par voie chimique sol-gel et dĂ©posĂ©es sur des substrats de verre par la technique de spin coating. La solution de dĂ©pĂŽt a Ă©tĂ© faite Ă partir des sels demĂ©taux de chlorure de cuivre (II) (CuCl2), chlorure de zinc (II) (ZnCl2), chlorure dâĂ©tain (IV) (SnCl4), et de thiourĂ©e (CS (NH2)2) dissous dans un mĂ©lange dâĂ©thanol/eau distillĂ©e comme solvant. Les Ă©tudes de diffraction de rayons X (DRX) ont rĂ©vĂ©lĂ©es la formation dâune structure kesterite avec des pics correspondant aux directions (112), (220), (312). Le spectre Raman a indiquĂ© la prĂ©sence dâun pic kesterite principal Ă la position 336 cm-1 et lâexistence dâun pic Ă©trange Ă la position 424 cm-1 pouvant correspondre au composĂ© binaire Cu2S. Les couches minces de CZTS ont montrĂ©s une transmission dâenviron 39% et une absorption optique au dessus de 104 cm-1 avec une Ă©nergie de gap de 2.34 eV. Ainsi les films de CZTS peuvent ĂȘtre synthĂ©tisĂ©s par une technique simple telle que le spin coating, mais des amĂ©liorations doivent ĂȘtre faites au niveau des propriĂ©tĂ©s des films en choisissant une tempĂ©rature et un temps de recuit convenables aussi bien quâun environnement de recuit appropriĂ©.Mots-clĂ©s :Cu2ZnSnS4 , sol-gel, spin coating, absorbeur photovoltaĂŻque
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