TOPoS：I.第一个样本的调查设计和分析
TOPoS: I. Survey design and analysis of the first sample
摘要：
星系晕星金属丰度分布函数的弱金属尾包含了第一代恒星形成模式的重要信息。为了在观测上确定这一点，有必要观测大量极贫金属的恒星。我们在这里介绍关闭原始恒星调查（TOPoS），这是在VLT进行的ESO大型计划。该项目有{4}个主要目标：（i）了解低金属含量气体中低质量恒星的形成：确定[M/H]=-3.5以下晕的金属弱尾。特别是，我们的目标是确定临界金属丰度，即足以形成低质量恒星的最低金属丰度;（二）确定极端贫金属恒星中元素的相对丰度，这是大质量第一颗恒星的标志;（三）确定银河系形成时锂丰度的趋势;以及（iv）导出C-增强极贫金属星相对于正常极贫金属星的比例。SDSS观测到的大量恒星为极低金属丰度的恒星候选者提供了很好的样本。关闭颜色下降到幅度g=20的候选人选择从低分辨率光谱的SDSS通过自动化程序。X-Shooter具有执行必要的后续光谱学的潜力，为这些恒星的几种关键元素提供准确的金属丰度和丰度比。我们在这里提出的恒星参数的第一组恒星。这19颗恒星的铁丰度相对于太阳在-4.1 dex到-2.9 dex之间。两颗恒星有很高的径向速度，根据我们对它们运动学的估计，它们似乎与银河系有很小的联系，可能是从另一个星系吸积而来的。
英文摘要：
The metal-weak tail of the metallicity distribution function (MDF) of the Galactic Halo stars contains crucial information on the formation mode of the first generation of stars. To determine this observationally, it is necessary to observe large numbers of extremely metal-poor stars. We present here the Turn-Off Primordial Stars survey (TOPoS) that is conducted as an ESO Large Programme at the VLT. This project has {four} main goals: (i) to understand the formation of low-mass stars in a low-metallicity gas: determine the metal-weak tail of the halo MDF below [M/H]=-3.5. In particular, we aim at determining the critical metallicity, that is the lowest metallicity sufficient for the formation of low-mass stars; (ii) to determine the relative abundance of the elements in extremely metal-poor stars, which are the signature of the massive first stars; (iii) to determine the trend of the lithium abundance at the time when the Galaxy formed; and (iv) to derive the fraction of C-enhanced extremely metal-poor stars with respect to normal extremely metal-poor stars. The large number of stars observed in the SDSS provides a good sample of candidates of stars at extremely low metallicity. Candidates with turn-off colours down to magnitude g=20 were selected from the low-resolution spectra of SDSS by means of an automated procedure. X-Shooter has the potential of performing the necessary follow-up spectroscopy, providing accurate metallicities and abundance ratios for several key elements for these stars. We here present the stellar parameters of the first set of stars. The nineteen stars range in iron abundance between -4.1 and -2.9 dex relative to the Sun. Two stars have a high radial velocity and, according to our estimate of their kinematics, appear to be marginally bound to the Galaxy and are possibly accreted from another galaxy.
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