An ancient system hidden in the Galactic plane?
Federico Sestito, Emma Fernandez-Alvar, Rebecca Brooks, Emma Olson, Leticia Carigi, Paula Jofre, Danielle de Brito Silva, Camilla J.L. Eldridge, Sara Vitali, Kim A. Venn, Vanessa Hill, Anke Ardern-Arentsen, Georges Kordopatis, Nicolas F. Martin, Julio F. Navarro

TL;DR
This study analyzes very metal-poor stars near the Galactic plane, revealing a chemically homogeneous group likely originating from a single early accretion event, with implications for Galactic formation history.
Contribution
It identifies a distinct population of planar VMP stars with unique chemical signatures, supporting a scenario of early accretion of a single progenitor system, possibly named Loki.
Findings
Planar VMP stars show chemical patterns similar to dwarf galaxies.
No significant chemical differences between prograde and retrograde stars.
The population likely resulted from early accretion of a single system.
Abstract
We analyse high signal-to-noise ESPaDOnS/CFHT spectra of 20 very metal-poor stars (VMP; [Fe/H]~) in the solar neighbourhood (within kpc), selected to be on planar orbits with maximum heights kpc. The sample comprises 11 stars on prograde and 9 on retrograde orbits, all with relatively high eccentricities (0.5--0.9).Their chemical abundance patterns indicate enrichment from high-energy supernovae and hypernovae up to the Fe-peak, and contributions from fast-rotating massive stars and neutron star mergers for the neutron-capture elements. No significant chemical differences are found between prograde and retrograde stars. The [Sr, Ba, Eu/Fe] ratios resemble those of stars in classical dwarfs galaxies. Chemical dispersion and distance analyses further highlight the internal similarity of the sample and its separation from the bulk of the observed, non-planar halo…
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