Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy
Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy
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DOI：
10.1051/0004-6361/200911959
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发表时间：
2009-08-01
影响因子：
6.5
通讯作者：
Babusiaux, C.
中科院分区：
文献类型：
--
作者：
Aoki, W.;Arimoto, N.;Babusiaux, C.
上下文。人们对银河系周围矮小球状星系中的单个恒星进行了光度和光谱研究。其中的极端贫金属星是非常有价值的，因为它们应该记录了局部群的早期富集态。然而，我们对这些恒星的了解非常有限，因为需要用高分辨率光谱仪进行详细的化学丰度测量。为了限制矮小星系的形成和化学演化，研究了极度贫金属星的金属丰度和化学成分。基于斯巴鲁望远镜高色散谱的高分辨光谱(R=40000)，测定了六角矮球星系中六颗极度贫金属星([Fe/H]<；-2.5)的化学丰度。(1)由高分辨谱得到的Fe丰度与由低分辨谱中的Ca三重线估算的金属丰度符合得很好。我们的测量证实了之前从Ca三重态的估计中发现的缺乏[Fe/H]小于或类似于-3的恒星，尽管我们注意到，为了估计具有如此低金属丰度的恒星的真实比例，有必要对更大样本的恒星进行高分辨率光谱分析。(2)虽然有一颗天体显示出过量的镁元素(类似于银晕恒星)，但其余五颗恒星的镁/铁比值与太阳值相似。这是第一次在矮球星系中发现如此低金属含量的低镁铁比。在这五颗恒星中没有发现钙和钛过多的证据，尽管对这些元素的测量不太确定。文中还讨论了相对于银晕恒星产生低镁铁比的可能机制。(3)在4个天体中，Ba的丰度不足，而其余的两个恒星表现出这种元素的大量和中等过剩。该星系中Ba的丰度分布与银晕中的分布相似，表明重元素的富集度可能是通过r过程开始的[Fe/H]。
Context. Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically. Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group. However, our understanding of these stars is very limited because detailed chemical abundance measurements are needed from high resolution spectroscopy.Aims. To constrain the formation and chemical evolution of dwarf galaxies, metallicity and chemical composition of extremely metal-poor stars are investigated.Methods. Chemical abundances of six extremely metal-poor ([Fe/H] < -2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy (R = 40 000) with the Subaru Telescope High Dispersion Spectrograph.Results. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H] less than or similar to -3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H]