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

Abstract

High-resolution optical spectra of 30 metal-poor stars selected from the Pristine survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters Teff and log g are determined using a Gaia DR2 colour-temperature calibration and surface gravity from the Stefan-Boltzmann equation. 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 other extremely metal-poor stars in the Galactic halo. Three 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 (σRV ≤ 0.2km 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 star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical halo [α/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.

A stellar stream remnant of a globular cluster below the metallicity floor

“…Finally, we computed bolometric corrections using a new grid of ATLAS models (Mucciarelli et al, in preparation). Our analysis of 30 similar metal-poor red giants with Gemini/GRACES spectra showed that the typical uncertainties in these stellar parameters are ±100 K in effective temperature and ±0.1 dex in log gravity 12 . We adopt these uncertainties here as well.…”

“…The 2-fibre (object+sky) mode was used, and the data initially reduced using the Gemini "Open-source Pipeline for ESPaDOnS Reduction and Analysis" tool, OPERA 45 . Additional steps were taken to improve the final 1D spectra, including improving the SNR in the overlapping order region by inversely weighing by the noise and improving the continuum normalization using an asymmetric sigma-clipping routine 12 . The full spectral range is ∼4,500 Å to ∼1 micron; however, light below ∼4,800 Å is severely limited by poor transmission through the very long optical fibre.…”

“…These stellar parameters are listed in Extended Data Table 3. We note that precision stellar parameters for extremely metal-poor benchmark stars on the red giant branch is currently a significant challenge 12,50,51 . The uncertainties on the stellar parameters and our previous analysis of similar stars and data imply that the quoted [Fe/H] metallicities have systematic uncertainties at the level of ±0.2 dex.…”

The Pristine survey -- XV. A CFHT ESPaDOnS view on the Milky Way halo and disc populations

“…For example, large samples of stars with a chemodynamical characterisation, like the one presented in this paper, can be used to check for possible associations with streams in the halo (e.g. Venn et al 2020;Kielty et al 2021). Along the same lines, it would be of interest to obtain high-resolution observations covering larger wavelength ranges (and spectral features of a wider variety of elements) for the two stars that chemo-dynamically stand out from the others (e.g., Pr_210.7513+12.7744 and Pr_255.8043+10.8443; see § 5) to investigate for possible associations with known structures/accretion events.…”

“…Spectroscopic follow-up of EMP star candidates is on-going. These observations are conducted both at medium and high-spectroscopic resolution (Caffau et al 2017;Starkenburg et al 2018;Bonifacio et al 2019;Venn et al 2020;Youakim et al 2017;Aguado et al 2019;Caffau et al 2020;Kielty et al 2021). The detailed chemical analysis of individual stars allows us to address three main threads of open issues related to stellar evolution and galaxy formation: (i) the nature and properties of the first stars, (ii) how and when the different components of the structure of the Milky Way assemble, and, finally, (iii) in the hierarchical galaxy formation paradigm, the mass and the degree of chemical evolution of the dwarf galaxy building blocks.…”