冷星铅和钍丰度测定的非LTE效应
Non-LTE effects on the lead and thorium abundance determinations for cool stars
摘要：
知道准确的铅丰度的贫金属恒星提供了限制铅生产机制在早期银河系。精确推导Th丰度允许恒星的核计时年龄测定。我们改进了基于非LTE线形成的恒星大气中Pb I和Th II线的计算，并通过一系列恒星参数评估偏离LTE对Pb和Th丰度测定的影响。提出了Pb I和Th II的综合模型原子。从LTE的偏离导致系统耗尽的总吸收的Pb I线和正的丰度校正。非LTE消除了太阳和陨石铅丰度之间的差异。使用Holweger & Mueller（1974）太阳模型大气层，log eps（Pb，non-LTE）= 2.09。我们修正了强r过程增强（r-II）恒星CS 31082-001和HE 1523-0901以及Roederer等人（2010）恒星样本的Pb和Eu丰度。我们的研究结果提供了强有力的证据，普遍的Pb/Eu相对r-过程产额在银河系演化过程中。-2.3<[Fe/H]< -1.4的恒星的Pb/Eu比平均比r-II恒星高0.51倍，这表明Pb的s-过程合成早在银河系金属丰度达到[Fe/H] =-2.3时就开始了。-1.4<[Fe/H]< -0.59的AGB星的平均Pb/Eu比值接近太阳值，与Travaglio等人（2001）的预言一致，即[Fe/H] ~-1的AGB星对太阳s核的Pb贡献最大。非LTE导致Th II线减弱。总的来说，当在非LTE计算中考虑与H1原子的碰撞时，丰度校正不超过+0.2dex。
英文摘要：
Knowing accurate Pb abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving Th abundances permits a nucleo-chronometric age determination of the star. We improve the calculation of the Pb I and Th II lines in stellar atmospheres based on non-LTE line formation and evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters. Comprehensive model atoms for Pb I and Th II are presented. The departures from LTE lead to systematically depleted total absorption in the Pb I lines and positive abundance corrections. Non-LTE removes the discrepancy between the solar and the meteoritic Pb abundance. With the Holweger & Mueller (1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001 and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results provide strong evidence for universal Pb/Eu relative r-process yields during course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars suggesting that the s-process synthesis of Pb started as early as the time when Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the -1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened Th II lines. Overall, the abundance correction does not exceed +0.2 dex when collisions with H I atoms are taken into account in non-LTE calculations.
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