A Survey of Proper-Motion Stars. XIV. Spectroscopic Binaries among Metal-poor Field Blue Stragglers
A Survey of Proper-Motion Stars. XIV. Spectroscopic Binaries among Metal-poor Field Blue Stragglers
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DOI：
10.1086/324233
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发表时间：
2001-12
期刊：
影响因子：
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通讯作者：
B. Carney;D. Latham;J. B. Laird;C. Grant;J. Morse
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文献类型：
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作者：
B. Carney;D. Latham;J. B. Laird;C. Grant;J. Morse
我们总结的结果，从一个程序的监测径向速度的10个贫金属，高速场星的颜色是0.01至0.13等蓝比主序星的可比金属丰度球状星团。两个候选的光环蓝掉队者（BD +72 94和BD +40 1166）没有显示速度变化的迹象，（HD 84937）只显示出微弱的变化迹象，（BD +25 1981）似乎是一个非常长周期的双星，（BD -12 2669、HD 97916、HD 106516、BD +51 1817、G66-30和G202-65）是单线光谱双星，周期从167天到844天不等。没有轨道解的四个候选者的速度范围从15.9年到19.0年不等。它们的轨道偏心率都很低，e < 0.30，而ε e ε = 0.11。六个双星轨道中有五个轨道的离心率非常低，平均离心率为0.07。我们重新分析了普雷斯顿和Sneden的速度数据，并对其中10颗[Fe/H] ≤-0.6的“蓝贫金属”恒星导出了与它们类似的轨道解。我们证实了他们的结论，即双星的频率很高;如果我们只包括[Fe/H] ≤-0.6的确定双星，我们发现47 ± 10%。我们对周期大于20天的7个双星的轨道解都有低的偏心率，e ≤ 0.26，ε e = 0.11。这些轨道特征与Ba II、CH、次巨星CH和矮碳星非常相似，表明它们的形成过程中涉及了质量转移。在我们的计划中，有五颗公布了锂丰度的双星，都被发现是缺乏的（还有一颗铍）。相比之下，三颗明显的单星中有两颗已经公布了锂的丰度，并且没有显示出缺乏。在我们的计划中的六个双星和普雷斯顿和斯奈登研究的七个类似系统的质量函数与它们看不见的同伴一致，都是M = 0.55 M的白色，轨道倾角随机。把我们所有的观察和其他人一起，我们认为，结果是一致的，所有领域的蓝离散是长周期和低偏心率的双星系统，主星缺乏锂和二级恒星是正常质量的白色矮星。所有这些属性都暗示了一个蓝离散形成模型，涉及传质。在这两个项目中的13颗恒星中，有6颗的s-过程元素丰度是可用的，没有明显的增强迹象，这表明施主星星是一颗第一次上升的红巨星。对于轨道周期最长（1307天）的星星CS 22956-028，s过程丰度的增强已经被报道。这颗星星可能是Luck和Bond提出的次巨星CH类的前身。
We summarize the results from a program of monitoring the radial velocities of 10 metal-poor, high-velocity field stars whose colors are 0.01 to 0.13 mag bluer than main-sequence turnoffs of comparable-metallicity globular clusters. Two of the candidate halo blue stragglers (BD +72 94 and BD +40 1166) show no signs of velocity variability, one (HD 84937) shows only weak signs of variability, one (BD +25 1981) appears to be a very long-period binary, and six (BD -12 2669, HD 97916, HD 106516, BD +51 1817, G66-30, and G202-65) are single-lined spectroscopic binaries, with periods ranging from 167 to 844 days. Velocity coverage for the four candidates without orbital solutions ranges from 15.9 to 19.0 years. The orbital eccentricities are all low, e < 0.30 and ⟨e⟩ = 0.11. Five of the six binary orbits have very low eccentricities, with ⟨e⟩ = 0.07. We have reanalyzed the velocity data from Preston & Sneden and have derived orbital solutions similar to theirs for 10 of the spectroscopic binaries among their "blue metal-poor" stars with [Fe/H] ≤ -0.6. We confirm their conclusion that the binary frequency is high; we find 47 ± 10% if we include only the definite binaries with [Fe/H] ≤ -0.6. Our orbital solutions for the seven binaries with periods longer than 20 days all have low eccentricities, with e ≤ 0.26 and ⟨e⟩ = 0.11. These orbital characteristics are very similar to the Ba II, CH, subgiant CH, and dwarf carbon stars, suggesting that mass transfer has been involved in their formation. Of the five binary stars in our program with published abundances of lithium, all have been found to be deficient (and one in beryllium as well). In contrast, two of the three apparently single stars have published lithium abundances and show no deficiency. The mass functions for the six binaries in our program and seven similar systems studied by Preston & Sneden are consistent with their unseen companions all being white dwarfs with M ≈ 0.55 M⊙ and random orbital inclinations. Taking all of our observations and those of others together, we argue that the results are consistent with all field blue stragglers being binary systems with long periods and low eccentricities, the primary stars being deficient in lithium and the secondary stars being normal-mass white dwarfs. All these properties are suggestive of a blue-straggler formation model that involves mass transfer. For six of the 13 stars in the two programs for which s-process elemental abundances are available, no signs of enhancement are discernible, suggesting that the donor star was a first-ascent red giant. For the star with the longest orbital period (1307 days), CS 22956-028, s-process abundance enhancements have been reported. This star may be a precursor to the subgiant CH class, as suggested by Luck & Bond.