An ancient system hidden in the Galactic plane?

Abstract We analyse high signal-to-noise ESPaDOnS/CFHT spectra of 20 very metal-poor stars (VMP; [Fe/H] <−2.0) in the solar neighbourhood (within ∼2 kpc), selected to be on planar orbits with maximum heights ≲ 4 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 population. Applying the same kinematical selection to another homogeneous dataset yields consistent results, confirming that this group of planar VMP stars exhibit peculiar chemical properties distinct from those of the observed halo and other known Galactic structures. These findings suggest that the stars formed in an environment that experienced a homogeneous chemical evolution akin to that of dwarf galaxies. A plausible scenario, supported by cosmological zoom-in simulations, is the early accretion of a single system whose subsequent dynamical evolution naturally produced stars on both prograde and retrograde planar orbits. If this interpretation is correct, we tentatively refer to this putative progenitor as Loki. However, comparisons with other planar VMP stars spanning a wider range of chemo-dynamical properties indicate that multiple accretion events likely contributed to this diverse population orbiting close to the Galactic plane.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2026-03-21
DOI
https://doi.org/10.1093/mnras/stag563
Primary Topic
Stellar, planetary, and galactic studies
Type
article
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article

An ancient system hidden in the Galactic plane?

G. Kordopatis, Federico Sestito, Leticia Carigi, P. Jofré et al.
Monthly Notices of the Royal Astronomical Society
Stellar, planetary, and galactic studies
article

An ancient system hidden in the Galactic plane?

G. Kordopatis, Federico Sestito, Leticia Carigi, P. Jofré, Else Starkenburg, C. Lardo, Tobias Buck, Romain Lucchesi, Camilla J. L. Eldridge, A Amayo, Emma Fernández-Alvar, Anke Arentsen, Julio Navarro, K. A. Venn, S. Vitali, Pascale Jablonka, Nicolas F Martin, Vanessa Hill, Patricia B Tissera, Danielle de Brito Silva, Rebecca Brooks, Emma Olson
article en

Abstract

Abstract We analyse high signal-to-noise ESPaDOnS/CFHT spectra of 20 very metal-poor stars (VMP; [Fe/H] <−2.0) in the solar neighbourhood (within ∼2 kpc), selected to be on planar orbits with maximum heights ≲ 4 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 population. Applying the same kinematical selection to another homogeneous dataset yields consistent results, confirming that this group of planar VMP stars exhibit peculiar chemical properties distinct from those of the observed halo and other known Galactic structures. These findings suggest that the stars formed in an environment that experienced a homogeneous chemical evolution akin to that of dwarf galaxies. A plausible scenario, supported by cosmological zoom-in simulations, is the early accretion of a single system whose subsequent dynamical evolution naturally produced stars on both prograde and retrograde planar orbits. If this interpretation is correct, we tentatively refer to this putative progenitor as Loki. However, comparisons with other planar VMP stars spanning a wider range of chemo-dynamical properties indicate that multiple accretion events likely contributed to this diverse population orbiting close to the Galactic plane.

Monthly Notices of the Royal Astronomical Society
Uppsala University (SE), Centre National de la Recherche Scientifique (FR), University of Hertfordshire (GB), Universidad de La Laguna (ES), Pontificia Universidad Católica de Chile (CL), Universidad Autónoma de la Ciudad de México (MX), University of Groningen (NL), University of Victoria (CA), Heidelberg University (DE), University of Cambridge (GB), Observatoire de Paris (FR), Observatoire astronomique de Strasbourg (FR), Galaxies, Etoiles, Physique et Instrumentation (FR), Osservatorio Astrofisico di Torino (IT), Max Planck Institute for Astronomy (DE), Institute of Astronomy (RU), Observatoire de la Côte d’Azur (FR), Instituto de Astrofísica de Canarias (ES), Bologna Research Area (IT), Astronomical Institute of the Slovak Academy of Sciences (SK), University of Florence (IT), École Polytechnique Fédérale de Lausanne (CH), Université de Strasbourg (FR), Universidad Nacional Autónoma de México (MX), Universidad Diego Portales (CL), University of Bologna (IT)
Openalex Percentile: Top 10%
Stellar, planetary, and galactic studies
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