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تسجيل مستخدم جديدChemo-dynamics of outer halo dwarf stars, including textit{Gaia}-Sausage and textit{Gaia}-Sequoia candidates
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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,$gsim+1.0$. 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 low orbital energy textit{Gaia}-Sequoia stars. A lower metallicity knee is tentatively identified in the textit{Gaia}-Sequoia high orbital 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.
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