J. R. Shi

A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars ?

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.

Abundances of Na, Mg and Al in nearby metal-poor stars

To determine the population membership of nearby stars we explore abundance results obtained for the light neutron- rich elements 23 Na and 27 Al in a small sample of moderately metal-poor stars. Spectroscopic observations are limited to the solar neighbourhood so that gravities can be determined from H parallaxes, and the results are confronted with those for a separate sample of more metal-poor typical halo stars. Following earlier investigations, the abundances of Na, Mg and Al have been derived from NLTE statistical equilibrium calculations used as input to line profile synthesis. Compared with LTE the abundances require systematic corrections, with typical values of +0.05 for (Mg/Fe), −0.1 for (Na/Fe) and +0.2 for (Al/Fe) in thick disk stars where (Fe/H) ∼− 0.6. In more metal-poor halo stars these values reach +0.1, −0.4, and +0.5, respectively, differences that can no longer be ignored. After careful selection of a clean subsample free from suspected or known binaries and peculiar stars, we find that (Na /Mg) and (Al/Mg), in combination with (Mg/Fe), space velocities and stellar evolutionary ages, make possible an individual discrimina- tion between thick disk and halo stars. At present, this evidence is limited by the small number of stars analyzed. We identify a gap at (Al/Mg) ∼− 0.15 and (Fe/H) ∼− 1.0 that isolates stars of the thick disk from those in the halo. A similar separation occurs at (Na/Mg) ∼− 0.4. We do not confirm the age gap between thin and thick disk found by Fuhrmann. Instead we find an age boundary between halo and thick disk stars, however, with an absolute value of 14 Gyr that must be considered as preliminary. While the stellar sample is by no means complete, the resulting abundances indicate the necessity to revise current models of chemical evolution and/or stellar nucleosynthesis to allow for an adequate production of neutron-rich species in early stellar generations.

Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars

Aims. Parameters for 55 nearby metal-poor stars are determined using high-resolution spectroscopy. Together with similar data taken from a recent analysis, they are used to show trends of their Galactic evolution with stellar [Fe/H] or [Mg/H] abundances. The separation of abundance ratios between disk and halo stars is used as a basic criterion for population membership. Methods. After careful selection of a clean subsample free of suspected or known binaries and peculiar stars, abundances of Mg, Na and Al are based on NLTE kinetic equilibrium calculations applied to spectrum synthesis methods. Results. The relation between [Na/Mg] and [Fe/H] is a continuous enrichment through all three Galactic populations spanning a range of values between a metal-poor plateau at [Na/Mg] = −0.7 and solar values. [Al/Mg] displays a step-like difference between stars of the Galactic halo with [Al/Mg] ∼− 0.45 and the two disk populations with [Al/Mg] ∼ +0.10. [Al/Mg] ratios, together with the [Mg/Fe] ratios, asymmetric drift velocities V, and stellar evolutionary ages, make possible the individual discrimination between stars of the thick disk and the halo. At present, this evidence is limited by the small number of stars, and by the theoretical and empirical uncertainties of stellar age determinations, but it achieves a high significance. Conclusions. While the stellar sample is not complete with respect to space volume, the resulting abundances indicate the necessity to revise current models of chemical evolution to allow for an adequate production of Al in early stellar generations.

Statistical equilibrium of silicon in the solar atmosphere

Aims. The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. Methods. High-resolution solar spectra were used to determine solar log g f eSi values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metalpoor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. Results. The statistical equilibrium of Si i turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Conclusions. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental f -values of Garz corrected for the Becker et al.’s measurement is 7.52 ± 0.05. Combined with an extended line sample with selected NIST f -values, the solar abundance is 7.52 ± 0.06, with a nearly perfect ionisation equilibrium of Δ log e� (Siii/Sii) = −0.01.

Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

A project of a spectroscopic surveyof Galactic structure and evolutionwith a Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) is presented. The spectro- scopic survey consists of two observational modes for various targets in our Galaxy. One is a major survey of the Milky Way aimed at a systematic study of the stellar abundance and Galactic chemical evolution through low resolution (R = 1000 − 2000) spectroscopy. Another is a follow-up observation with medium resolution (R = 10000) spectrographs aimed at detailedstudies of the selected stars with differentchemicalcomposition,kinematics and dynamics. The universe consists of millions of galaxies, in which there is a special one - the Milky Way - where all we stay. The Milky Way is also the subject most concerned by astronomers since it links our knowledge of stellar evolution and some important problems of formation of the universe. It can be used as an ideal laboratory for the studies of large scale structure of the Universe and gas distribution. We can observe more easily the various objects in the Milky Way than those in other galaxies. The modern study of the Milky Way by the method of star counting was started in late 19th century. Other galaxies were identified to be similar stellar systems as the Milky Way at beginning of 20th century. Considering that the Milky Way is a typical spiral galaxy in which our solar system is located, it is the best sample with which various theoretical models can be checked when we study the formation and evo- lution of the galaxies. The structure and chemical evolution of a galaxy can be represented by the different distribution and motion of the stars with different metallicities. There was no exciting progress in the field of Galactic study before 1980s due to the lack of sufficient observational data. Thanks to the rapid development of high resolution spectrographs and some large scale

Sodium abundances in nearby disk stars

We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The NLTE effects are more pronounced in metal-poor stars since the statistical equilibrium is dominated by collisions of which at least the electronic component is substantially reduced. The resulting influence on the determination of sodium abundances is in a direction opposite to that found previously for Mg and Al. The NLTE corrections are about −0.1 in thick-disk stars with (Fe/H) ∼− 0.6. Our (Na/Fe) abundance ratios are about solar for thick- and thin-disk stars. The increase in (Na/ Fe) as af unction of (Fe/H) for metal-rich stars found by Edvardsson et al. (1993) is confirmed. Our results suggest that sodium yields increase with the metallicity, and quite large amounts of sodium may be produced by AGB stars. We find that (Na/Fe) ratios, together with either (Mg/Fe) ratio, kinematic data or stellar evolutionary ages, make possible the individual discrimination between thin- and thick-disk membership.

LAMOST Spectroscopic Survey of the Galactic Anticentre (LSS-GAC): target selection and the first release of value-added catalogues

As a major component of the LAMOST Galactic surveys, the LAMOST Spectroscopic Survey of the Galactic Anti-centre (LSS-GAC) aims to survey a significant volume of the Galactic thin/thick discs and halo for a contiguous sky area of over 3,400 deg

Automatic determination of stellar atmospheric parameters and construction of stellar spectral templates of the Guoshoujing telescope (LAMOST)

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The LAMOST stellar parameter pipeline at Peking University – lsp3

centred on the Galactic anti-centre (

Systematic Non-LTE study of the −2.6 ≤ [Fe/H] ≤ 0.2 F and G dwarfs in the solar neighborhood - I. Stellar atmosphere parameters

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