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Now showing items 1 - 9 of 9

  • Confronting the Gaia and NLTE spectroscopic parallaxes for the FGK stars

    Sitnova, Tatyana   Mashonkina, Lyudmila   Pakhomov, Yury  

    The understanding of the chemical evolution of the Galaxy relies on the stellar chemical composition. Accurate atmospheric parameters is a prerequisite of determination of accurate chemical abundances. For late type stars with known distance, surface gravity (log g) can be calculated from well-known relation between stellar mass, T-eff, and absolute bolometric magnitude. This method weakly depends on model atmospheres, and provides reliable log g. However, accurate distances are available for limited number of stars. Another way to determine log g for cool stars is based on ionisation equilibrium, i.e. consistent abundances from lines of neutral and ionised species. In this study we determine atmospheric parameters moving step-bystep from well-studied nearby dwarfs to ultra-metal poor (UMP) giants. In each sample, we select stars with the most reliable T-eff based on photometry and the distance-based log g, and compare with spectroscopic gravity calculated taking into account deviations from local thermodinamic equilibrium (LTE). After that, we apply spectroscopic method of log g determination to other stars of the sample with unknown distances.
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  • Confronting the Gaia and NLTE spectroscopic parallaxes for the FGK stars

    Sitnova, Tatyana   Mashonkina, Lyudmila   Pakhomov, Yury  

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  • Impact of NLTE on research of early chemical enrichment of the dwarf galaxies

    Mashonkina, Lyudmila   Jablonka, Pascale   North, Pierre   Sitnova, Tatyana  

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  • Review: progress in NLTE calculations and their application to large data-sets

    Mashonkina, Lyudmila  

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  • Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

    Mashonkina, Lyudmila   Gehren, Thomas   Shi, Jianrong   Korn, Andreas   Grupp, Frank  

    Non-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and beta Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.
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  • Atomic data necessary for the non-LTE analysis of stellar spectra

    Mashonkina, Lyudmila  

    The determination of physical parameters and chemical abundances of stars based on non-local thermodynamic equilibrium (non-LTE) line formation requires a large amount of accurate atomic data on energy levels, photoionization cross-sections, oscillator strengths, inelastic collision cross-sections, and related data. In this paper, we briefly review astrophysical problems where non-LTE modelling leads to a better interpretation of observed data due to the use of advanced atomic data and where atomic data are still missing or uncertain. For the statistical equilibrium of atoms in cool stars, a top priority problem is the efficiency of inelastic collisions with hydrogen atoms.
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  • Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

    Mashonkina, Lyudmila   Gehren, Thomas   Shi, Jianrong   Korn, Andreas   Grupp, Frank  

    Non-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and beta Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.
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  • NLTE Line Formation for MgI and MgII in the Atmospheres of B-A-F-G-K Stars

    Alexeeva, Sofya   Ryabchikova, Tatiana   Mashonkina, Lyudmila   Hu, Shaoming  

    Non-local thermodynamical equilibrium (NLTE) line formation for Mg I and Mg II lines is considered in classical 1D LTE model atmospheres of the Sun and 17 stars with reliable atmospheric parameters and in a broad range of spectral types: 3900 K <=3D T-eff <=3D 17,500 K, 1.1 <=3D log g <=3D 4.7, and -2.6 <=3D [Fe/H] <=3D +0.4. We find that, for each star, NLTE leads to smaller line-to-line scatter. For 10 stars, NLTE leads to consistent abundances of Mg I and Mg II, while the difference in LTE abundance varies between -0.21 and +0.23. dex. We obtain an abundance discrepancy between Mg I and Mg II in two very metal-poor stars, HD 140283 and HD 84937. The origin of these abundance differences remains unclear. Our standard NLTE modeling predicts Mg I emission lines at 7.736, 11.789, 12.224, and 12.321 mu m in the atmospheres with T-eff <=3D 7000 K. We reproduce well the Mg I 12.2 and 12.3 mu m emission lines in Procyon. However, for the Sun and three K-giants, the predicted Mg I emission lines are too weak compared with the observations. For stars with 7000 K <=3D T-eff <=3D 17,500 K, we recommend the Mg II 3848, 3850, 4384, 4390, 4427, and 4433. A lines for Mg abundance determinations even at the LTE assumption due to their small NLTE effects. The Mg I 4167, 4571, 4702, 5528, 5167, 5172, and 5183 angstrom lines can be safely used in the LTE analysis of stars with 7000 K <=3D T-eff <=3D 8000 K. For the hotter stars, with T-eff from 8000 K to 9500 K, the NLTE effects are minor only for Mg I 4167, 4702, and 4528 angstrom.
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  • Impact of NLTE on determinations of atmospheric parameters and chemical abundances of very metal-poor stars

    Mashonkina, Lyudmila   Sitnova, Tatyana   Pakhomov, Yuri   Ryabchikova, Tatyana  

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