低金属丰度的NIR Ca II三重态--在经典矮星系中寻找极低金属丰度的恒星
The NIR Ca II triplet at low metallicity - Searching for extremely low-metallicity stars in classical dwarf galaxies
摘要：
近红外Ca II三重吸收线已被证明是本星系群中单个红巨星分支恒星的定量光谱学的重要工具，提供了更好地了解银河系和矮星系中恒星的金属丰度，从而有机会限制其化学演化过程。这个领域中一个有趣的谜题是，使用这种技术在银河系周围的经典矮星系中发现了[Fe/H]=-3以下的极度贫金属恒星。问题是这些恒星是否真的不存在，或者用于研究这些系统的经验Ca II三重态方法是否偏向于低金属量区域。在这里，我们提出的合成光谱分析的Ca II三重态，这是集中在更好地了解低金属量巨星的光谱测量的结果。当[Fe/H]<-2时，我们的结果开始严重偏离广泛使用的线性经验校准。我们为Ca II三重态研究提供了一种新的校准，其在-0.5<[Fe/H]<-4时有效。随后，我们应用这种新的校准到目前的数据集，并建议经典矮星系是不是那么缺乏极低金属丰度的恒星，因为以前认为。
英文摘要：
The NIR Ca II triplet absorption lines have proven to be an important tool for quantitative spectroscopy of individual red giant branch stars in the Local Group, providing a better understanding of metallicities of stars in the Milky Way and dwarf galaxies and thereby an opportunity to constrain their chemical evolution processes. An interesting puzzle in this field is the significant lack of extremely metal-poor stars, below [Fe/H]=-3, found in classical dwarf galaxies around the Milky Way using this technique. The question arises whether these stars are really absent, or if the empirical Ca II triplet method used to study these systems is biased in the low-metallicity regime. Here we present results of synthetic spectral analysis of the Ca II triplet, that is focused on a better understanding of spectroscopic measurements of low-metallicity giant stars. Our results start to deviate strongly from the widely-used and linear empirical calibrations at [Fe/H]<-2. We provide a new calibration for Ca II triplet studies which is valid for -0.5<[Fe/H]<-4. We subsequently apply this new calibration to current data sets and suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought.
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