The Pristine survey – XII. Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy

C. Kielty, K. Venn, F. Sestito, E. Starkenburg, N. Martin, D. Aguado, A. Arentsen, S. Fabbro, J. I. González Hernández, V. Hill, P. Jablonka, C. Lardo, L. Mashonkina, J. Navarro, C. Sneden, G. Thomas, K. Youakim, Spencer Bialek, R. Sánchez-Janssen
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引用次数: 9

Abstract

High-resolution optical spectra of 30 metal-poor stars selected from the \textit{Pristine} survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters T$_{\rm eff}$ and $\log g$ are determined using Gaia DR2 parallaxes and MIST/MESA metal-poor isochrones. GRACES spectra are used to determine chemical abundances (or upper-limits) for 20 elements (Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu). These stars are confirmed to be metal-poor ([Fe/H]$<-2.5$), with higher precision than from earlier medium-resolution analyses. The chemistry for most targets is similar to that of other extremely metal-poor stars in the Galactic halo. Two stars near [Fe/H]$=-3.0$ have unusually low Ca and high Mg, suggestive of contributions from few SN~II, where alpha-element formation through hydrostatic nucleosynthesis was more efficient. Three new carbon-enhanced metal-poor stars are also identified (two CEMP-s and one potential CEMP-no star), when our chemical abundances are combined with carbon from previous medium-resolution analyses. The GRACES spectra also provide precision radial velocities ($\sigma_{\rm RV}\le0.2$ km s$^{-1}$) for dynamical orbit calculations with the Gaia DR2 proper motions. Most of our targets are dynamically associated with the Galactic halo; however, five stars with [Fe/H]$<-3$ have planar-like orbits, including one retrograde planar star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical high [$\alpha$/Fe] ratios for their metallicities, whereas two are [Mg/Fe]-deficient, and one is a new CEMP-s candidate. These results are discussed in terms of the formation and early chemical evolution of the Galaxy.
原始调查-十二。双子座-格雷斯对银河系中新发现的极贫金属恒星的化学动力学研究
根据双子座天文台GRACES光谱仪的观测结果,展示了从\textit{原始}调查中选择的30颗金属贫乏恒星的高分辨率光谱。恒星参数T $_{\rm eff}$和$\log g$是使用盖亚DR2视差和MIST/MESA等时线确定的。grace光谱用于确定20种元素(Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu)的化学丰度(或上限)。这些恒星被证实是金属贫乏的([Fe/H] $<-2.5$),比早期的中等分辨率分析精度更高。大多数目标的化学性质与银河晕中其他极度缺乏金属的恒星相似。在[Fe/H] $=-3.0$附近的两颗恒星有异常低的Ca和高的Mg,暗示了SN II的贡献,其中通过流体静力核合成的α元素形成更有效。当我们的化学丰度与之前中等分辨率分析的碳相结合时,还发现了三颗新的碳增强金属贫星(两颗cmp -s星和一颗潜在的cmp -无星)。GRACES光谱还提供了精确的径向速度($\sigma_{\rm RV}\le0.2$ km s $^{-1}$),用于盖亚DR2固有运动的动态轨道计算。我们的大多数目标都与银河晕动态地联系在一起;然而,有五颗[Fe/H] $<-3$的恒星有类似平面的轨道,包括一颗逆行的平面恒星。另外5颗恒星与Gaia-Sequoia吸积事件动态一致;三种金属丰度具有典型的高[$\alpha$ /Fe]比率,而两种是[Mg/Fe]缺乏,一种是新的cmp -s候选者。这些结果将从银河系的形成和早期化学演化的角度进行讨论。
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