X-Shooter GTO：EMP候选样品的化学分析
X-Shooter GTO: Chemical analysis of a sample of EMP candidates
摘要：
内容：极贫金属星（EMP）是非常罕见的天体，其大气层中保存着星系演化早期阶段化学成分的化石记录。发现这些天体并确定其化学成分为这些早期阶段提供了重要的限制。目的：使用精心设计的选择方法，我们从斯隆数字巡天的低分辨率光谱中选择了一个候选EMP星的样本，并在VLT用X-Shooter观察它们，以确认它们的金属丰度并确定尽可能多的元素丰度。方法：通过一维，平面平行，流体静力学模型大气的X-Shooter光谱进行分析。使用CO 5 BOLD流体动力学模拟计算颗粒效应的校正。结果：所有候选星都被确认为EMP星，证明了我们的选择方法的效率在0.5 dex以内。这个样本的化学成分与较亮的样本的化学成分是相容的，这表明星系晕中的恒星混合得很好。结论这些观测结果表明，在有限的时间内，从SDSS中选出的EMP候选恒星中观测大约100颗的大样本是可行的。这样的样本量将特别使我们能够证实或反驳在[Fe/H] ~-3.5附近的星系晕金属丰度分布函数中存在垂直下降。
英文摘要：
Context: Extremely metal-poor stars (EMP) are very rare objects that hold in their atmospheres the fossil record of the chemical composition of the early phases of Galactic evolution. Finding these objects and determining their chemical composition provides important constraints on these early phases. Aims: Using a carefully designed selection method, we chose a sample of candidate EMP stars from the low resolution spectra of the Sloan Digital Sky Survey and observed them with X-Shooter at the VLT to confirm their metallicities and determine abundances for as many elements as possible. Method: The X-Shooter spectra are analysed by means of one-dimensional, plane-parallel, hydrostatic model atmospheres. Corrections for the granulation effects are computed using CO5BOLD hydrodynamical simulations. Results: All the candidates are confirmed to be EMP stars, proving the efficiency of our selection method within about 0.5 dex. The chemical composition of this sample is compatible with those of brighter samples, suggesting that the stars in the Galactic halo are well mixed. Conclusions These observations show that it is feasible to observe, in a limited amount of time, a large sample of about one hundred stars among EMP candidates selected from the SDSS. Such a size of sample will allow us, in particular, to confirm or refute the existence of a vertical drop in the Galactic Halo metallicity distribution function around [Fe/H] ~ -3.5.
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