THE CH(G) INDEX AS A NEW CRITERION FOR SELECTING RED GIANT STARS
THE CH(G) INDEX AS A NEW CRITERION FOR SELECTING RED GIANT STARS
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DOI：
10.1088/0004-637x/765/2/156
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发表时间：
2013-03
期刊：
影响因子：
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通讯作者：
Y. Chen;Gang Zhao;K. Carrell;Jie Zhao;K. Tan
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文献类型：
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作者：
Y. Chen;Gang Zhao;K. Carrell;Jie Zhao;K. Tan
利用Sloan数字巡天（SDSS）光谱测量了球状星团M3中演化恒星的CH G带（CH（G））指数。研究发现，在已知光谱中金属丰度的情况下，利用CH（G）指数对（g − r）0图，可以从渐近巨星分支（AGB）和红色水平分支（RHB）等演化恒星的污染中筛选出红巨星分支（RGB）星。当这个图应用于具有类似金属丰度的场巨星时，我们建立了CH（G）= 1.625（g-r）0 − 1.174（g-r）20 − 0.934的校准。这个方法被[Fe/H]<$−2.3的恒星证实，其中球状星团M15和M92的成员星的光谱可以在SDSS数据库中找到。因此，我们使用0.4 <$（g-r）0 <$1.0的红巨星，将这种校准扩展到[Fe/H]<$-3.0到[Fe/H]<$0.0的每个金属丰度区间。根据金属性相关的校准，当−3.0 < [Fe/H] δ −1.2时，CH（G）= 1.625（g-r）0 − 1.174（g-r）20 + 0.060[Fe/H] − 0.830;当−1.2 < [Fe/H] δ < 0.0时，CH（G）= 0.953（g-r）0 − 0.655（g-r）20 + 0.060[Fe/H] − 0.650。该校准对SDSS光谱数据集是有效的，并且它们不能盲目地应用于其他数据集。通过两次定标，排除了大量的污染星（AGB和RHB星），因此通过选择±0.05等以内的恒星获得了清晰的红巨星样本。该样本在网上公布，预计这个大而干净的RGB恒星样本将提供有关银河系形成和演化的新信息。
We have measured the CH G band (CH(G)) index for evolved stars in the globular cluster M3 based on the Sloan Digital Sky Survey (SDSS) spectroscopic survey. It is found that there is a useful way to select red giant branch (RGB) stars from the contamination of other evolved stars such as asymptotic giant branch (AGB) and red horizontal branch (RHB) stars by using the CH(G) index versus (g − r)0 diagram if the metallicity is known from the spectra. When this diagram is applied to field giant stars with similar metallicity, we establish a calibration of CH(G) = 1.625(g − r)0 − 1.174(g − r)20 − 0.934. This method is confirmed by stars with [Fe/H] ∼ −2.3 where spectra of member stars in globular clusters M15 and M92 are available in the SDSS database. We thus extend this kind of calibration to every individual metallicity bin ranging from [Fe/H] ∼ −3.0 to [Fe/H] ∼ 0.0 by using field red giant stars with 0.4 ⩽ (g − r)0 ⩽ 1.0. The metallicity-dependent calibrations give CH(G) = 1.625(g − r)0 − 1.174(g − r)20 + 0.060[Fe/H] − 0.830 for −3.0 < [Fe/H] ⩽ −1.2 and CH(G) = 0.953(g − r)0 − 0.655(g − r)20 + 0.060[Fe/H] − 0.650 for −1.2 < [Fe/H] < 0.0. The calibrations are valid for the SDSS spectroscopic data set, and they cannot be applied blindly to other data sets. With the two calibrations, a significant number of the contaminating stars (AGB and RHB stars) were excluded and thus a clear sample of red giant stars is obtained by selecting stars within ±0.05 mag of the calibration. The sample is published online and it is expected that this large and clean sample of RGB stars will provide new information on the formation and evolution of the Galaxy.