The role of binaries in the enrichment of the early Galactic halo
The role of binaries in the enrichment of the early Galactic halo
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DOI：
10.1051/0004-6361/201527409
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发表时间：
2015-11
影响因子：
6.5
通讯作者：
T. Hansen;J. Andersen;J. Andersen;B. Nordström;B. Nordström;T. Beers;V. Placco;J. Yoon;L. Buchhave;L. Buchhave
中科院分区：
文献类型：
--
作者：
T. Hansen;J. Andersen;J. Andersen;B. Nordström;B. Nordström;T. Beers;V. Placco;J. Yoon;L. Buchhave;L. Buchhave
对贫金属晕星的详细光谱研究突出了碳增强贫金属（CEMP）星在理解银河系早期碳的产生和喷射以及在大爆炸后形成的第一批恒星中识别CEMP星的祖先方面的重要作用。最近的工作也根据绝对碳丰度A（C）将CEMP恒星分为高C带和低C带，主要分别由双星和单星组成。我们的目标是确定的频率和轨道参数的CEMP-s的恒星，表现出强烈的增强与S-过程中的中子捕获元素之间的二进制系统。这使我们能够测试是否有必要和足够的地方质量转移从一个双伴侣来解释他们的戏剧性的碳过剩。22颗恒星中有18颗呈现清晰的轨道运动，产生82+-10%的双星频率，而4颗恒星似乎是单星（18+-10%）。因此，我们确认CEMP-s恒星的双星频率比正常的贫金属巨星高得多，但不是以前声称的100%。确定了11个双星的安全轨道和6个长周期系统（P > 3,000天）的临时轨道，并讨论了轨道循环时间尺度。传统的情况下，从一个前AGB双星伴星的局部质量转移似乎占大多数CEMP-s恒星的化学成分。然而，在一些单一的CEMP-s恒星中，多余的C和s-过程元素显然是通过外部（遥远的）来源转移到它们的纳塔尔云。这一发现对我们理解早期银河系晕和一些高红移DLA系统中的碳富集以及极端贫金属AGB恒星的质量损失具有重要意义。节录。
Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high- and low-C bands, mostly populated by binary and single stars, respectively. Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82+-10%, while four stars appear to be single (18+-10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for 11 of the binaries and provisional orbits for six long-period systems (P > 3,000 days), and orbital circularisation time scales are discussed. The conventional scenario of local mass transfer from a former AGB binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their natal clouds by an external (distant) source. This finding has important implications for our understanding of carbon enrichment in the early Galactic halo and some high-redshift DLA systems, and of the mass loss from extremely metal-poor AGB stars. Abridged.