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

Collin L. Kielty, Kim A. Venn, Federico Sestito, Else Starkenburg, Nicolas F. Martin, David S. Aguado, Anke Arentsen, Sébastien Fabbro, Jonay I. González Hernández, Vanessa Hill, Pascale Jablonka, Carmela Lardo, Lyudmila I. Mashonkina, Julio F. Navarro, Chris Sneden, Guillaume F. Thomas, Kris Youakim, Spencer Bialek, Rubén Sánchez-Janssen

Published 2020-12-07Version 1

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.