http://arxiv.org/abs/1909.11969
High resolution Keck HIRES and Gemini GRACES spectra are combined with Gaia DR2 photometry and astrometry to re-examine the low-metallicity, kinematically interesting dwarf stars studied by Stephens & Boesgaard (2002). These stars, spanning a metallicity range of $-1.5>$ [Fe/H] $>-3$, were originally selected as outer halo dwarf stars because of their large apocentric radii, large distances from the Milky Way plane and/or highly elliptical or retrograde orbits. Using additional chemical and kinematic criteria, we select a subsample of six stars (test cases) to redetermine stellar parameters and abundances using new model atmospheres and updated atomic data. We confirm two of the six stars in our sample to be very metal-poor, with [Fe/H]$\sim -2.3$ (G158-100, G037-037) and find two other stars to be Mg-poor at intermediate metallicities [Fe/H]$\sim-1.5$ (G184-007 and G189-050). The latter signature is suggestive of origins in an accreted dwarf galaxy. Gaia DR2 data is used to redetermine the orbital parameters for the entire SB02 sample. We find nine stars to be dynamically coincident with the \textit{Gaia}-Enceledas satellite merger, including one very low metallicity star (G238-030) ([Fe/H]$\sim-3.6$). Another 17 stars are found to be dynamically coincident with the \textit{Gaia}-Sequoia accretion event, including one very metal-poor star near [Fe/H] $=-3.5$. Both metal-poor stars have low masses and isochrone ages older than 10 Gyr. A knee in [$\alpha$/Fe] is found in the \textit{Gaia}-Enceladus associated stars, near [Fe/H]$\sim-1.6$, while the location of an [$\alpha$/Fe] is less clear in the \textit{Gaia}-Sequoia sample. If the metal-poor stars in these samples are true members of the \textit{Gaia}-Enceledas and \textit{Gaia}-Sequoia remnants, they present unique opportunities to probe the low metallicity tail and early star formation history of these systems.
S. Monty, K. Venn, J. Lane, et. al.
Fri, 27 Sep 19
42/64
Comments: 20 pages, 17 figures, submitted to MNRAS. Comments welcome
You must be logged in to post a comment.