7,364 research outputs found
A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars
A comprehensive model atom for Fe with more than 3000 energy levels is
presented. As a test and first application of this model atom, Fe abundances
are determined for the Sun and five stars with well determined stellar
parameters and high-quality observed spectra. Non-LTE leads to systematically
depleted total absorption in the Fe I lines and to positive abundance
corrections in agreement with the previous studies, however, the magnitude of
non-LTE effect is smaller compared to the earlier results. Non-LTE corrections
do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient
stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars
HD 84937 and HD 122563, respectively, depending on the assumed efficiency of
collisions with hydrogen atoms. Based on the analysis of the Fe I/Fe II
ionization equilibrium in these two stars, we recommend to apply the Drawin
formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe II
lines, non-LTE corrections do not exceed 0.01 dex in absolute value. The solar
non-LTE abundance obtained from 54 Fe I lines is 7.56+-0.09 and the abundance
from 18 Fe II lines varies between 7.41+-0.11 and 7.56+-0.05 depending on the
source of the gf-values. Thus, gf-values available for the iron lines are not
accurate enough to pursue high-accuracy absolute abundance determinations.
Lines of Fe I give, on average, a 0.1 dex lower abundance compared to those of
Fe II lines for HD 61421 and HD 102870, even when applying a differential
analysis relative to the Sun. A disparity between Fe I and Fe II points to
problems of stellar atmosphere modelling or/and effective temperature
determination.Comment: 19 pages, 8 figures, online material, accepted by A&
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
Large Galactic stellar surveys and new generations of stellar atmosphere
models and spectral line formation computations need to be subjected to careful
calibration and validation and to benchmark tests. We focus on cool stars and
aim at establishing a sample of 34 Gaia FGK Benchmark Stars with a range of
different metallicities. The goal was to determine the effective temperature
and the surface gravity independently from spectroscopy and atmospheric models
as far as possible. Fundamental determinations of Teff and logg were obtained
in a systematic way from a compilation of angular diameter measurements and
bolometric fluxes, and from a homogeneous mass determination based on stellar
evolution models. The derived parameters were compared to recent spectroscopic
and photometric determinations and to gravity estimates based on seismic data.
Most of the adopted diameter measurements have formal uncertainties around 1%,
which translate into uncertainties in effective temperature of 0.5%. The
measurements of bolometric flux seem to be accurate to 5% or better, which
contributes about 1% or less to the uncertainties in effective temperature. The
comparisons of parameter determinations with the literature show in general
good agreements with a few exceptions, most notably for the coolest stars and
for metal-poor stars. The sample consists of 29 FGK-type stars and 5 M giants.
Among the FGK stars, 21 have reliable parameters suitable for testing,
validation, or calibration purposes. For four stars, future adjustments of the
fundamental Teff are required, and for five stars the logg determination needs
to be improved. Future extensions of the sample of Gaia FGK Benchmark Stars are
required to fill gaps in parameter space, and we include a list of suggested
candidates.Comment: Accepted by A&A; 34 pages (printer format), 14 tables, 13 figures;
language correcte
Phase field theory of interfaces and crystal nucleation in a eutectic system of fcc structure: I. Transitions in the one-phase liquid region
The published version of this Article can be accessed from the link below - Copyright @ 2007 American Institute of PhysicsThe phase field theory (PFT) has been applied to predict equilibrium interfacial properties and nucleation barrier in the binary eutectic system Ag-Cu using double well and interpolation functions deduced from a Ginzburg-Landau expansion that considers fcc (face centered cubic) crystal symmetries. The temperature and composition dependent free energies of the liquid and solid phases are taken from CALculation of PHAse Diagrams-type calculations. The model parameters of PFT are fixed so as to recover an interface thickness of approximately 1 nm from molecular dynamics simulations and the interfacial free energies from the experimental dihedral angles available for the pure components. A nontrivial temperature and composition dependence for the equilibrium interfacial free energy is observed. Mapping the possible nucleation pathways, we find that the Ag and Cu rich critical fluctuations compete against each other in the neighborhood of the eutectic composition. The Tolman length is positive and shows a maximum as a function of undercooling. The PFT predictions for the critical undercooling are found to be consistent with experimental results. These results support the view that heterogeneous nucleation took place in the undercooling experiments available at present. We also present calculations using the classical droplet model classical nucleation theory (CNT) and a phenomenological diffuse interface theory (DIT). While the predictions of the CNT with a purely entropic interfacial free energy underestimate the critical undercooling, the DIT results appear to be in a reasonable agreement with the PFT predictions.This work has been supported by the Hungarian Academy of Sciences under Contract No. OTKA-K-62588 and by the ESA PECS Contract Nos. 98005, 98021, and 98043
Core compressor exit stage study, 2
A total of two three-stage compressors were designed and tested to determine the effects of aspect ratio on compressor performance. The first compressor was designed with an aspect ratio of 0.81; the other, with an aspect ratio of 1.22. Both compressors had a hub-tip ratio of 0.915, representative of the rear stages of a core compressor, and both were designed to achieve a 15.0% surge margin at design pressure ratios of 1.357 and 1.324, respectively, at a mean wheel speed of 167 m/sec. At design speed the 0.81 aspect ratio compressor achieved a pressure ratio of 1.346 at a corrected flow of 4.28 kg/sec and an adiabatic efficiency of 86.1%. The 1.22 aspect ratio design achieved a pressure ratio of 1.314 at 4.35 kg/sec flow and 87.0% adiabatic efficiency. Surge margin to peak efficiency was 24.0% with the lower aspect ratio blading, compared with 12.4% with the higher aspect ratio blading
New Abundances for Old Stars - Atomic Diffusion at Work in NGC 6397
A homogeneous spectroscopic analysis of unevolved and evolved stars in the
metal-poor globular cluster NGC 6397 with FLAMES-UVES reveals systematic trends
of stellar surface abundances that are likely caused by atomic diffusion. This
finding helps to understand, among other issues, why the lithium abundances of
old halo stars are significantly lower than the abundance found to be produced
shortly after the Big Bang.Comment: 8 pages, 7 colour figures, 1 table; can also be downloaded via
http://www.eso.org/messenger
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