332 research outputs found
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&
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 (4–32 years). The mean maximum knee flexion was 35° (0°–90°) in type I deficiencies and 118° (90°–140°) 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
Modeling of Alpha Cen and Procyon using VLTI observations
We present a novel approach to model the nearby stars Alpha Cen A & B and
Procyon A using asteroseismic and interferometric constraints. Using the VINCI
instrument installed at the VLT Interferometer (VLTI), the angular diameters of
the Alpha Centauri system were measured with a relative precision of 0.2% and
0.6%, respectively. From these values, we derive linear radii of R[A] = 1.224
+/- 0.003 R_sun and R[B] = 0.863 +/- 0.005 R_sun. These radii are in excellent
agreement with the models of Thevenin et al. (2002), that use asteroseismic
frequencies as constraints (Bouchy & Carrier 2001; Bouchy & Carrier 2002). With
the same instrument, we also measured the angular diameter of Procyon A. Using
the Hipparcos parallax, we obtain a linear radius of 2.048 +/- 0.025 R_sun. We
use this result together with spectroscopic and photometric constraints to
model this star with the CESAM code. We also computed the adiabatic oscillation
spectrum of our model of Procyon A, giving a mean large frequency separation of
Delta nu_0 = 54.8 uHz, in agreement with the seismic observations by Martic et
al. (2001). Our model favours a mass around 1.4 M_sun for Procyon A.Comment: 8 pages, 5 figures, poster presented at the JD 12 of the IAU General
Assembly, Sydney 200
Th Ages for Metal-Poor Stars
With a sample of 22 metal-poor stars, we demonstrate that the heavy element
abundance pattern (Z > 55) is the same as the r-process contributions to the
solar nebula. This bolsters the results of previous studies that there is a
universal r-process production pattern. We use the abundance of thorium in five
metal-poor stars, along with an estimate of the initial Th abundance based on
the abundances of stable r-process elements, to measure their ages. We have
four field red giants with errors of 4.2 Gyr in their ages and one M92 giant
with an error of 5.6 Gyr, based on considering the sources of observational
error only. We obtain an average age of 11.4 Gyr, which depends critically on
the assumption of an initial production ratio of Th/Eu of 0.496. If the
Universe is 15 Gyr old, then the initial Th/Eu value should be 0.590, in
agreement with some theoretical models of the r-process.Comment: 26 pages, to be published in Ap
s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-element
content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass
s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are
derived using the spectral synthesis technique from high-resolution spectra.
The N-stars analyzed are of nearly solar metallicity and show moderate
s-element enhancements, similar to those found in S stars, but smaller than
those found in the only previous similar study (Utsumi 1985), and also smaller
than those found in supergiant post-AGB stars. This is in agreement with the
present understanding of the envelope s-element enrichment in giant stars,
which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the
AGB phase. We compare the observational data with recent -process
nucleosynthesis models for different metallicities and stellar masses. Good
agreement is obtained between low mass AGB star models (M < 3 M_o) and
s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction
is the main source of neutrons for the s-process; a moderate spread, however,
must exist in the abundance of 13C that is burnt in different stars. By
combining information deriving from the detection of Tc, the infrared colours
and the theoretical relations between stellar mass, metallicity and the final
C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this
work are intrinsic, thermally-pulsing AGB stars; their abundances are the
consequence of the operation of third dredge-up and are not to be ascribed to
mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap
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