从矮恒星的Mg同位素看星系晕的形成时标
A formation timescale of the Galactic halo from Mg isotopes in dwarf stars
摘要：
我们确定镁同位素丰度的金属贫矮星的星系晕，揭示渐近巨星分支（AGB）恒星核合成的星系晕的发病和限制其形成的时间尺度。我们使用凯克天文台的HIRES摄谱仪（R~10^5和200 < S/N < 300）观测到了8个新的晕K矮星样本，其金属丰度范围为-1.9 < [Fe/H] < -0.9和4200 < $T_{eff}$（K）< 4950。我们得到镁同位素丰度的光谱合成三个MgH功能，并比较我们的结果与银河系化学演化模型。利用目前的样本，我们几乎将文献中确定的镁同位素贫金属恒星的数量增加了一倍。新的数据使我们能够确定金属丰度时，$^{26}$镁丰度开始变得重要，$[Fe/H] \sim -1.4 \pm 0.1$。使用$[Fe/H] > -1.4$的数据比以前的化学演化模型预测值稍高（1-3 $σ$），表明富中子同位素的产率可能更高。我们的结果只使用AGB星富集建议形成的星系晕的时间尺度约为0.3 Gyr，但也考虑到超新星富集，时间尺度形成的上限约为1.5 Gyr。
英文摘要：
We determine magnesium isotopic abundances of metal poor dwarf stars from the galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star nucleossynthesis in the galactic halo and constrain the timescale of its formation. We observed a sample of eight new halo K dwarfs in a metallicity range of -1.9 < [Fe/H] < -0.9 and 4200 < $T_{eff}$(K) < 4950, using the HIRES spectrograph at the Keck Observatory (R~10^5 and 200 < S/N < 300). We obtain magnesium isotopic abundances by spectral synthesis on three MgH features and compare our results with galactic chemical evolution models. With the current sample, we almost double the number of metal poor stars with Mg isotopes determined from the literature. The new data allow us to determine the metallicity when the $^{26}$Mg abundances start to became important, $[Fe/H] \sim -1.4 \pm 0.1$. The data with $[Fe/H] > -1.4$ are somewhat higher (1-3 $σ$) than previous chemical evolution model predictions, indicating perhaps higher yields of the neutron-rich isotopes. Our results using only AGB star enrichment suggest a timescale for formation for the galactic halo of about 0.3 Gyr, but considering also supernova enrichment, the upper limit for the timescale formation is about 1.5 Gyr.
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