Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Boötes I
Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Boötes I
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DOI：
10.1051/0004-6361/201322796
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发表时间：
2014-01
影响因子：
6.5
通讯作者：
M. Ishigaki;W. Aoki;N. Arimoto;S. Okamoto
中科院分区：
文献类型：
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作者：
M. Ishigaki;W. Aoki;N. Arimoto;S. Okamoto
上下文最近在银河系（MW）周围发现的超暗矮星系包含极贫金属的恒星，可能代表了MW低金属量组分的构建块。其中，牧夫座一号矮球状星系是特别感兴趣的，因为它的唯一的老恒星人口。这个星系中单个恒星的详细化学成分是理解宇宙中最古老星系的形成和化学演化及其在建立MW晕中的作用的关键。目标。先前对牧夫座I的化学丰度的研究表明，不同作者之间的元素丰度存在差异，因此尚未建立其化学富集历史的一致图片。在目前的工作中，我们独立地确定了牧夫座I中六颗红巨星的化学成分，其中一些与以前的研究中分析的成分重叠。基于导出的丰度，我们重新检查元素丰度的趋势和散射，并与MW场晕星和MW中的其他矮球状星系进行比较。方法.安装在斯巴鲁望远镜上的高色散摄谱仪获得了恒星样本的高分辨率光谱。测定了样品星的C、Na、α、Fe峰和中子俘获元素等12种元素的丰度。的丰度结果进行了比较，在现场MW晕星以前获得的丰度分析技术类似于本研究。结果我们确认Boo-094的金属丰度较低（[Fe/H] =-3.4）。除了这颗星星之外，比锌轻的元素的丰度比（[X/Fe]）通常是均匀的，在其他五颗恒星的金属丰度平均值周围有很小的分散（−2.7 −2.7），表明碳没有显著的增强。[Mg/Fe]和[Ca/Fe]比值几乎不变，随着[Fe/H]的增加有适度降低的趋势，略低于场晕星。牧夫座I星的[Sr/Fe]和[Sr/Ba]比值也倾向于低于晕星。结论.我们的研究结果表明，轻于锌的元素的[X/Fe]比值的小分散表明，这些丰度在该星系中前几代恒星的喷出物中是均匀的。相对于场晕星，平均[Mg/Fe]和[Ca/Fe]比值较低，而这些丰度比值与某些金属丰度[Fe/H] < −2的更明亮的矮球状星系相似，这可以解释为牧夫座I的星星形成至少持续到Ia型超新星开始对该星系的化学富集做出贡献。
Context. Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW) contain extremely metal-poor stars, and might represent the building blocks of low-metallicity components of the MW. Among them, the Bootes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population. Detailed chemical compositions of individual stars in this galaxy are a key to understanding formation and chemical evolution in the oldest galaxies in the Universe and their roles in building up the MW halo. Aims. Previous studies of the chemical abundances of Bootes I show discrepancies in elemental abundances between different authors, and thus a consistent picture of its chemical enrichment history has not yet been established. In the present work, we independently determine chemical compositions of six red giant stars in Bootes I, some of which overlap with those analyzed in the previous studies. Based on the derived abundances, we re-examine trends and scatters in elemental abundances and make comparisons with MW field halo stars and other dwarf spheroidal galaxies in the MW. Methods. High-resolution spectra of a sample of stars were obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12 elements, including C, Na, α, Fe-peak, and neutron capture elements, were determined for the sample stars. The abundance results were compared to those in field MW halo stars previously obtained using an abundance analysis technique similar to the present study. Results. We confirm the low metallicity of Boo-094 ([Fe/H] = −3.4). Except for this star, the abundance ratios ([X/Fe]) of elements lighter than zinc are generally homogeneous with small scatter around the mean values in the metallicities spanned by the other five stars (−2.7 −2.7 show no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost constant with a modest decreasing trend with increasing [Fe/H] and are slightly lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be lower in the Bootes I stars than in the halo stars. Conclusions. Our results of small scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these abundances were homogeneous among the ejecta of prior generation(s) of stars in this galaxy. The lower mean [Mg/Fe] and [Ca/Fe] ratios relative to the field halo stars and the similarity in these abundance ratios with some of the more luminous dwarf spheroidal galaxies at metallicities [Fe/H] < −2 can be interpreted as star formation in Bootes I having lasted at least until Type Ia supernovae started to contribute to the chemical enrichment in this galaxy.