The low-metallicity, kinematically interesting dwarf stars studied by Stephens \& Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry, and updated model atmospheres and atomic line data. New stellar parameters are determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar Evolution Database isochrones. These are in excellent agreement with spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$; however, large disagreements are found for stars with [Fe/H]$\le-2$, with offsets as large as $\Delta$T$_{\rm eff}\sim+500$ K and $\Delta$log\,$g\sim+1.0$. A subset of six stars (test cases) are analysed ab initio using high resolution spectra with Keck HIRES and Gemini GRACES. This sub-sample is found to include two $\alpha$-challenged dwarf stars, suggestive of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the entire SB02 sample are re-determined using \textit{Gaia} DR2 data. We find 11 stars that are dynamically coincident with the \textit{Gaia}-Sausage accretion event and another 17 with the \textit{Gaia}-Sequoia event in action space. Both associations include low-mass, metal-poor stars with isochrone ages older than 10 Gyr. Two dynamical subsets are identified within \textit{Gaia}-Sequoia. When these subsets are examined separately, a common knee in [$\alpha$/Fe] is found for the \textit{Gaia}-Sausage and high energy \textit{Gaia}-Sequoia stars. A lower metallicity knee is tentatively identified in the \textit{Gaia}-Sequoia low energy stars. If the metal-poor dwarf stars in these samples are true members of the \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia events, then they present a unique opportunity to probe the earlier, more pristine, star formation histories of these systems.

中文翻译：

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外晕矮星的化学动力学，包括 Gaia-Sausage 和 Gaia-Sequoia 候选星

Stephens \& Boesgaard (2002, SB02) 研究的低金属丰度、运动学上有趣的矮星使用 Gaia DR2 天体测量法以及更新的模型大气和原子线数据重新检查。新的恒星参数是根据 Gaia DR2 视差距离和达特茅斯恒星演化数据库等时线确定的。对于 [Fe/H]$>-2$ 的恒星，这些与光谱确定的恒星参数非常吻合；然而，发现 [Fe/H]$\le-2$ 的恒星存在很大的分歧，偏移量高达 $\Delta$T$_{\rm eff}\sim+500$ K 和 $\Delta$log \,$g\sim+1.0$。使用 Keck HIRES 和 Gemini GRACES 的高分辨率光谱从头开始分析六颗星的子集（测试案例）。这个子样本被发现包括两颗受到 $\alpha$ 挑战的矮星，暗示起源于低质量，吸积矮星系。整个 SB02 样品的轨道参数使用 \textit{Gaia} DR2 数据重新确定。我们发现 11 颗星与 \textit{Gaia}-Sausage 吸积事件动态重合，另外 17 颗星与动作空间中的 \textit{Gaia}-Sequoia 事件重合。这两个协会都包括等时线年龄大于 10 Gyr 的低质量、贫金属恒星。在 \textit{Gaia}-Sequoia 中确定了两个动态子集。当分别检查这些子集时，发现 \textit{Gaia}-Sausage 和高能量 \textit{Gaia}-Sequoia 恒星的 [$\alpha$/Fe] 中的共同拐点。在 \textit{Gaia}-Sequoia 低能量星中暂时确定了较低的金属丰度膝盖。如果这些样本中的贫金属矮星是 \textit{Gaia}-Sausage 和 \textit{Gaia}-Sequoia 事件的真正成员，