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《中国天文和天体物理学报(英文版)》 2003-04
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Non-LTE Analysis of the Sodium Abundance of Metal- Poor Stars in the Galactic Disk and Halo

Yoichi Takeda, Gang Zhao, Masahide Takada-Hidai, Yu-Qin Chen, Yu-ji Saito and Hua-Wei Zhang 1 Komazawa University, Komazawa, Setagaya, Tokyo 154-8525, Japan; takedayi@cc.nao.ac.jp 2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 3 Liberal Arts Education Center, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan 4 Department of Physics, Faculty of Science, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan 5 Department of Astronomy, School of Physics, Peking University, Beijing 100871 6 CAS-Peking University Joint Beijing Astrophysical Center, Beijing 100871  
We performed an extensive non-LTE analysis of the neutral sodium lines of Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F-K covering a wide metallicity range (-4 (?) [Fe/H] (?) +0.4), using our own data as well as data collected from the literature. For comparatively metal-rich disk stars (-1(?) [Fe/H] (?) +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed [Na/Fe]- 0 with an "upturn" (i.e., a shallow/broad dip around -0.5 (?) [Fe/H](?) 0) as already reported in previous studies. For the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines subject to considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly "subsolar" [Na/Fe] values down to--0.4 (with a somewhat large scatter of-±0.2 dex) on the average at the typical halo metallicity of [Fe/H] --2, followed by a rise again to a near-solar ratio of [Na/Fe] - 0 at the very metal-poor regime [Fe/H]--3 to -4. These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.
【Fund】: National Natural Science Foundation of China.
【CateGory Index】: P148
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