H. W. Zhang

Non-LTE line formation for heavy elements in four very metal-poor stars

Aims. Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor (VMP) stars (−2.15 ≥ [Fe/H] ≥− 2.66). For two of them, HD 84937 and HD 122563, the fraction of the odd isotopes of Ba derived for the first time. Methods. Determination of an effective temperature, surface gravity, and element abundances was based on non-local thermodynamic equilibrium (non-LTE) line formation and analysis of high-resolution (R ∼ 60 000 and 90 000) high signal-to-noise (S/N ≥ 200) observed spectra. A model atom for H i is presented. An effective temperature was obtained from the Balmer Hα and Hβ line wing fits. The surface gravity was calculated from the Hipparcos parallax if available and the non-LTE ionization balance between Ca i and Ca ii. Based on the hyperfine structure affecting the Ba ii resonance line λ 4554, the fractional abundance of the odd isotopes of Ba was derived from a requirement that Ba abundances from the resonance line and subordinate lines of Ba ii must be equal. Results. For each star, non-LTE leads to a consistency of Teff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from the ionization balance between Ca i and Ca ii. For each star with a known trigonometric surface gravity, non-LTE abundances from the lines of two ionization stages, Ca i and Ca ii, agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for all investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, fodd, equals 0.43 ± 0.14. The latter can serve as an observational constraint on r-process models. The lower Eu/Ba ratio and fodd = 0.22 ± 0.15 found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star.

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

The LAMOST stellar parameter pipeline at Peking University – lsp3

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

centred on the Galactic anti-centre (

A non-local thermodynamic equilibrum study of scandium in the Sun

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Lithium abundances in metal-poor stars

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Potassium abundances in nearby metal-poor stars

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SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

), and obtain

NLTE analysis of the solar potassium abundance (Research Note)

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Atmospheric extinction coefficients and night sky brightness at the Xuyi Observation Station

3700 -- 9000 low resolution (