We present a new opacity sampling model atmosphere code, named MAFAGS-OS. This code, designed for stars reaching from A0 down to G9 on a solar and metal poor main sequence and up to an evolutionary ...stage represented by the turnoff is introduced in its basic input physics and modelling techniques. Fe I bound-free cross-sections of Bautista (1997) are used and convection is treated according to Canuto & Mazzitelli (1991). An alpha cm-parameter for the efficiency of convection of 0.82 is used as determined by Bernkopf (1998) from stellar evolution requirements. Within the process of opacity sampling, special attention is drawn to the matter of line selection. We show that a selection criterion, in which lines are chosen by their opacity weighted relative to the continuous background opacity, is useful and valid. The solar model calculated using this new code is shown to fit the measured solar flux distribution. It is also tested against the measured solar colours and leads to U - B = 0.21 and B - V = 0.64, in good agreement with observation. Comparison with measured centre-to-limb continuum data show only small improvement with respect to opacity-sampling type model atmospheres. This is the first of a series of 2 papers. Paper II will deal with the matter of temperature determination using Balmer lines and the infrared-flux method; furthermore it will present three "standard" stars analysed using this new model.
We present atmospheric parameters for 51 nearby F and G dwarf and subgiant stars uniformly distributed over the -2.60 < Fe/H < +0.20 metallicity range that is suitable for the Galactic chemical ...evolution research. Lines of iron in the two ionization stages, Fe sub(I) and Fe sub(II), were used to derive a homogeneous set of effective temperatures, surface gravities, iron abundances, and microturbulence velocities. Our spectroscopic analyses took advantage of employing high-resolution (R > or =, slanted 60,000) Shane/Hamilton and Canada-France-Hawaii Telescope/ESPaDOnS observed spectra and non-LTE (NLTE) line formation for Fe sub(I) and Fe sub(II) in the classical one-dimensional model atmospheres. The spectroscopic method was tested in advance with the 20 benchmark stars, for which there are multiple measurements of the infrared flux method effective temperature and their Hipparcos parallax error is less than 10%. We found NLTE abundances from lines of Fe sub(I) and Fe sub(II) to be consistent within 0.06 dex for every benchmark star, when applying a scaling factor of S sub(H) = 0.5 to the Drawinian rates of inelastic Fe+H collisions. The obtained atmospheric parameters were checked for each program star by comparing its position in the log g-T sub(eff) plane with the theoretical evolutionary track of given metallicity and alpha -enhancement in the Yi et al. grid. Our final effective temperatures lie exactly in between the T sub(IRFM) scales of Alonso et al., and Casagrande et al., with a mean difference of +46 and -51 K, respectively. NLTE leads to higher surface gravity compared with that for LTE. The shift in log g is smaller than 0.1 dex for stars with Fe/H > or =, slanted -0.75, T sub(eff) < or =, slant 5750 K, or log g > or =, slanted 4.20. NLTE analysis is crucial for the very metal-poor turnoff and subgiant stars, for which the shift in log g between NLTE and LTE can be up to 0.5 dex. The obtained accurate atmospheric parameters will be used in the forthcoming papers to determine NLTE abundances of important astrophysical elements from lithium to europium and to improve observational constraints on the chemodynamical models of the Galaxy evolution.
Aims. We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for the two ions of iron and check the ionization equilibrium between Fe i and Fe ii in model atmospheres of the cool ...reference stars based on the best available complete model atom for neutral and singly-ionized iron. Methods. We present a comprehensive model atom for Fe with more than 3000 measured and predicted energy levels. As a test and first application of the improved model atom, iron abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. The efficiency of inelastic collisions with hydrogen atoms in the statistical equilibrium of iron is empirically estimated from inspection of their different influence on the Fe i and Fe ii lines in the selected stars. Results. 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 such corrections is smaller compared to the earlier results. These 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 over the whole range of stellar parameters that are considered. This study reveals two problems. The first one is that gf-values available for the Fe i and Fe ii lines are not accurate enough to pursue high-accuracy absolute stellar abundance determinations. For the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56 ± 0.09 and the mean 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. The second problem is that lines of Fe i give, on average, a 0.1 dex lower abundance compared with those of Fe ii lines for HD 61421 and HD 102870, even when applying a differential line-by-line analysis with regard to the Sun. A disparity between neutral atoms and first ions points to problems of stellar atmosphere modelling or/and effective temperature determination.
The influence of the new MAFAGS-OS opacity sampling model atmosphere and its changed thermal structure is tested on two of the major methods to determine stellar effective temperatures: the infrared ...flux method and the Balmer line method. Both methods are shown to be affected by the new model. How a change of the resonance broadening theory according to Ali & Griem (1965) improves the theoretical Balmer line spectrum of the Sun is studied. The new model together with the modified resonance broadening cross sections for hydrogen are then applied to three stars: Procyon, Groombridge 1830 and HD 19445. Stellar parameters measured for these stars lead to new determinations of masses, ages and spectroscopic distances for these objects. While Procyon's parameters are only slightly changed with respect to recent ODF-based analyses, the temperatures of Groombridge 1830 and HD 19445 are significantly increased, leading to higher masses and lower ages for these two stars. Compared to ODF-based studies, the paradox of Groombridge 1830 and HD 19445 having ages well above the age of the Universe is resolved and the determined spectroscopic parallaxes are in very good agreement with high precision HIPPARCOS astrometry.
Context.
Mixing-length theory is used to treat stellar convection. As a simulation in one-dimensional stellar atmospheres models, the mixing-length parameter
α
is calibrated from the Sun and then ...applied to other stars. However, there is no strong evidence to suggest that
α
should be the same for stars of different evolutionary stages.
Aims.
We evaluate the impact of the
α
value on the metallicity of different types of stars and investigate the correlation between the metallicity discrepancy (ΔFe∕H) and stellar parameters (
T
eff
, log
g
).
Methods.
We selected ten well-studied field stars and one open cluster of nine members for which high-resolution and high signal-to-noise spectra are available. The model atmospheres were calculated with the code MAFAGS-OS. We derived iron abundances from Fe
I
and Fe
II
lines both under local thermodynamic equilibrium and non-LTE conditions using a spectrum synthesis method. After deriving Fe/H for each line, we calculated ΔFe∕H with two different
α
values, fixed solar-calibrated
α
, and
α
obtained for each star individually. Finally, we investigated the correlation between ΔFe∕H caused by revised
α
with stellar parameters.
Results.
For FGK dwarf stars, the ΔFe∕H caused by the
α
correction is less than 0.02 dex, while for turn-off and giant stars, the ΔFe∕H values are no more than 0.03 dex, which are lower than typical uncertainties in metallicity. For main-sequence stars, ΔFe∕H versus
T
eff
and ΔFe∕H versus log
g
are well fit by linear relations.
We present here CAFE, the Calar Alto Fiber-fed Échelle spectrograph, a new instrument built at the Centro Astronomico Hispano Alemán (CAHA). CAFE is a single-fiber, high-resolution (R ~ 70 000) ...spectrograph, covering the wavelength range between 3650−9800 Å. It was built on the basis of the common design for Échelle spectrographs. Its main aim is to measure radial velocities of stellar objects up to V ~ 13−14 mag with a precision as good as a few tens of m s-1. To achieve this goal the design was simplified at maximum, removing all possible movable components, the central wavelength is fixed, as is the wavelength coverage; there is no filter wheel, etc. Particular care was taken with the thermal and mechanical stability. The instrument is fully operational and publically accessible at the 2.2 m telescope of the Calar Alto Observatory. In this article we describe (i) the design, summarizing its manufacturing phase; (ii) characterize the main properties of the instrument; (iii) describe the reduction pipeline; and (iv) show the results from the first light and commissioning runs. The preliminar results indicate that the instrument fulfills the specifications and can achieve the planned goals. In particular, the results show that the instrument is more efficient than anticipated, reaching a signal-to-noise of ~20 for a stellar object as faint as V ~ 14.5 mag in ~2700 s integration time. The instrument is a wonderful machine for exoplanetary research (by studying large samples of possible systems cotaining massive planets), galactic dynamics (highly precise radial velocities in moving groups or stellar associations), or astrochemistry.
Multi-Zonal computer generated holograms (MZ-CGHs) in combination with interferometric wavefront measurements are perfectly suited as optical adjustment tools - especially if the demands on the ...alignment accuracy are very high. After reviewing the basic idea for alignment with MZ-CGHs, we derive the analytic relation between the interferometrically observed tilt and power values and the associated lens placement errors, including estimates of the applied approximations. This analysis yields the parameters determining the principle sensitivity of the method. Subsequently, the achievable accuracy of large 6″ MZ-CGHs in practical application is tested with a series of different optical measurements which confirm the technical feasibility. The productive use of the technique will be presented in part II of the paper for different examples in the framework of the Euclid space telescope.
Weak lensing, which is the deflection of light by matter along the line of sight, has proven to be an efficient method for constraining models of structure formation and reveal the nature of dark ...energy. So far, most weak-lensing studies have focused on the shear field that can be measured directly from the ellipticity of background galaxies. However, within the context of forthcoming full-sky weak-lensing surveys such as Euclid , convergence maps (mass maps) offer an important advantage over shear fields in terms of cosmological exploitation. While it carry the same information, the lensing signal is more compressed in the convergence maps than in the shear field. This simplifies otherwise computationally expensive analyses, for instance, non-Gaussianity studies. However, the inversion of the non-local shear field requires accurate control of systematic effects caused by holes in the data field, field borders, shape noise, and the fact that the shear is not a direct observable (reduced shear). We present the two mass-inversion methods that are included in the official Euclid data-processing pipeline: the standard Kaiser & Squires method (KS), and a new mass-inversion method (KS+) that aims to reduce the information loss during the mass inversion. This new method is based on the KS method and includes corrections for mass-mapping systematic effects. The results of the KS+ method are compared to the original implementation of the KS method in its simplest form, using the Euclid Flagship mock galaxy catalogue. In particular, we estimate the quality of the reconstruction by comparing the two-point correlation functions and third- and fourth-order moments obtained from shear and convergence maps, and we analyse each systematic effect independently and simultaneously. We show that the KS+ method substantially reduces the errors on the two-point correlation function and moments compared to the KS method. In particular, we show that the errors introduced by the mass inversion on the two-point correlation of the convergence maps are reduced by a factor of about 5, while the errors on the third- and fourth-order moments are reduced by factors of about 2 and 10, respectively.
Context. Synthetic model atmosphere calculations are still the most commonly used tool when determining precise stellar parameters and stellar chemical compositions. Besides three-dimensional models ...that consistently solve for hydrodynamic processes, one-dimensional models that use an approximation for convective energy transport play the major role. Aims. We use modern Balmer-line formation theory as well as spectral energy distribution (SED) measurements for the Sun and Procyon to calibrate the model parameter α that describes the efficiency of convection in our 1D models. Convection was calibrated over a significant range in parameter space, reaching from F-K along the main sequence and sampling the turnoff and giant branch over a wide range of metallicities. This calibration was compared to theoretical evaluations and allowed an accurate modeling of stellar atmospheres. Methods. We used Balmer-line fitting and SED fits to determine the convective efficiency parameter α. Both methods are sensitive to the structure and temperature stratification of the deeper photosphere. Results. While SED fits do not allow a precise determination of the convective parameter for the Sun and Procyon, they both favor values significantly higher than 1.0. Balmer-line fitting, which we find to be more sensitive, suggests that the convective efficiency parameter α is ≈2.0 for the main sequence and quickly decreases to ≈1.0 for evolved stars. These results are highly consistent with predictions from 3D models. While the values on the main sequence fit predictions very well, measurements suggest that the decrease of convective efficiency as stars evolve to the giant branch is more dramatic than predicted by models.
PDF
